Apparatus and methods for ventilatory treatment

ABSTRACT

Disclosed is an apparatus for treating a respiratory disorder, configured to compute a measure of typical recent ventilation such that a rate of adjustment of the measure of typical recent ventilation is reduced as a measure of recent uncompensated leak increases. Also disclosed is an apparatus for treating a respiratory disorder, configured to compute a target ventilation from a product of a measure of typical recent ventilation and a target fraction, wherein the target fraction is dependent on the recent pressure support.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.61/623,643, filed 13 Apr. 2012, the disclosure of which is incorporatedherein in its entirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

THE NAMES OF PARTIES TO A JOINT RESEARCH DEVELOPMENT

Not Applicable

SEQUENCE LISTING

Not Applicable

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present technology relates to one or more of the diagnosis,treatment and amelioration of respiratory disorders, and to proceduresto prevent respiratory disorders. In particular, the present technologyrelates to medical devices, and their use for treating respiratorydisorders and for preventing respiratory disorders.

(2) Description of the Related Art

The respiratory system of the body facilitates gas exchange. The noseand mouth form the entrance to the airways of a patient.

The airways include a series of branching tubes, which become narrower,shorter and more numerous as they penetrate deeper into the lung. Theprime function of the lung is gas exchange, allowing oxygen to move fromthe air into the venous blood and carbon dioxide to move out. Thetrachea divides into right and left main bronchi, which further divideeventually into terminal bronchioles. The bronchi make up the conductingairways, and do not take part in gas exchange. Further divisions of theairways lead to the respiratory bronchioles, and eventually to thealveoli. The alveolated region of the lung is where the gas exchangetakes place, and is referred to as the respiratory zone. See West,Respiratory Physiology—the essentials.

A range of respiratory disorders exist.

Obstructive Sleep Apnea (OSA), a form of Sleep Disordered Breathing(SDB), is characterized by occlusion or obstruction of the upper airpassage during sleep. It results from a combination of an abnormallysmall upper airway and the normal loss of muscle tone in the region ofthe tongue, soft palate and posterior oropharyngeal wall during sleep.The condition causes the affected patient to stop breathing for periodstypically of 30 to 120 seconds duration, sometimes 200 to 300 times pernight. It often causes excessive daytime somnolence, and it may causecardiovascular disease and brain damage. The syndrome is a commondisorder, particularly in middle aged overweight males, although aperson affected may have no awareness of the problem. See U.S. Pat. No.4,944,310 (Sullivan).

Cheyne-Stokes Respiration (CSR) is a disorder of a patient's respiratorycontroller in which there are rhythmic alternating periods of waxing andwaning ventilation, causing repetitive de-oxygenation and re-oxygenationof the arterial blood. It is possible that CSR is harmful because of therepetitive hypoxia. In some patients CSR is associated with repetitivearousal from sleep, which causes severe sleep disruption, increasedsympathetic activity, and increased afterload. See U.S. Pat. No.6,532,959 (Berthon-Jones).

Periodic breathing disorders of central origin, such as Cheyne-Stokesrespiration, may occur together with upper airway obstruction.

The diagnosis of CSR usually involves conducting a sleep study andanalyzing the resulting polysomnography (“PSG”) data. In a fulldiagnostic PSG study, a range of biological parameters are monitoredthat typically include a nasal flow signal, measures of respiratoryeffort, pulse oximetry, sleeping position, and may include:electroencephalography (“EEG”), electrocardiography (“ECG”),electromyography (“EMG”) and electro-oculography (“EOG”). Breathingcharacteristics are also identified from visual features, thus allowinga clinician to assess respiratory function during sleep and evaluate anypresence of CSR. While the examination by a clinician is the mostcomprehensive method, it is a costly process and depends heavily uponclinical experience and understanding.

Systems

One known product used for treating sleep disordered breathing is the S9Sleep Therapy System, manufactured by ResMed.

Therapy

Nasal Continuous Positive Airway Pressure (CPAP) therapy has been usedto treat Obstructive Sleep Apnea (OSA). The hypothesis is thatcontinuous positive airway pressure acts as a pneumatic splint and mayprevent upper airway obstruction by pushing the soft palate and tongueforward and away from the posterior oropharyngeal wall.

Non-invasive ventilation (NIV) has been used to treat CSR, OHS, COPD, MDand Chest Wall disorders. In some cases of NIV, the pressure treatmentmay be controlled to enforce a target ventilation by measuring a tidalvolume or minute ventilation, for example, and controlling the measureof ventilation to satisfy the target ventilation. Servo-controlling ofthe measure of ventilation, such as by, a comparison of an instantaneousmeasure of ventilation and a long term measure of ventilation, may serveas a treatment to counteract CSR. In some such cases, the form of thepressure treatment delivered by an apparatus may be Pressure Supportventilation. Such a pressure treatment typically provides generation ofa higher level of pressure during inspiration (e.g., an IPAP) andgeneration of a lower level of pressure during expiration (e.g., anEPAP).

Patient Interface

The application of a supply of air at positive pressure to the entranceof the airways of a patient is facilitated by the use of a patientinterface, such as a nasal mask, full-face mask or nasal pillows. Arange of patient interface devices are known, however a number of themsuffer from being one or more of obtrusive, aesthetically undesirable,poorly fitting, difficult to use and uncomfortable especially when wornfor long periods of time or when a patient is unfamiliar with a system.Masks designed solely for aviators, as part of personal protectionequipment or for the administration of anaesthetics may be tolerable fortheir original application, but nevertheless be undesirablyuncomfortable to be worn for extended periods, for example, whilesleeping.

PAP Device

The air at positive pressure is typically supplied to the airway of apatient by a PAP device such as a motor-driven blower. The outlet of theblower is connected via a flexible delivery conduit to a patientinterface as described above.

BRIEF SUMMARY OF THE TECHNOLOGY

The present technology is directed towards providing medical devicesused in the detection, diagnosis, amelioration, treatment, or preventionof respiratory disorders having one or more of improved comfort, cost,efficacy, ease of use and manufacturability.

A first aspect of the present technology relates to apparatus used inthe detection, diagnosis, amelioration, treatment or prevention of arespiratory disorder.

Another aspect of the present technology relates to methods used in thedetection, diagnosis, amelioration, treatment or prevention of arespiratory disorder.

Aspects of the present technology provide methods for evaluating orassessing patient SDB events and/or ventilation, which may beimplemented in apparatus for assessment of ventilation or apparatus forgenerating a respiratory pressure treatment.

Aspects of the present technology provide methods and apparatus thatautomatically adjust the level of EPAP in order to counteract upperairway obstruction during respiratory pressure treatment of periodicbreathing.

One aspect of one form of the present technology comprises aservo-ventilator configured to control the pressure of a supply of airso as to achieve a target ventilation, which, in response to amisleading change in measured ventilation, for example as a result of asudden change in leak, reduces a rate of adjustment of the targetventilation.

One aspect of one form of the present technology comprises aservo-ventilator configured to: continuously compute a targetventilation such that the target ventilation rises more slowly as ameasure of recent uncompensated leak increases, and control the pressureof a supply of air so as to achieve the target ventilation.

One aspect of one form of the present technology comprises apparatus ormethods for treating a respiratory disorder that provide a measure oftypical recent ventilation that rises more slowly as a measure of recentuncompensated leak increases.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that provide a targetventilation whose rate of increase is bounded by an upper limit.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that provide a targetventilation that falls more swiftly as the stability of recent pressuresupport increases, so as to improve patient comfort.

These three most recently described aspects may be particularlyadvantageous when used in combination.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that adjust a value ofexpiratory positive airway pressure (EPAP) according to the duration ofa detected apnea or hypopnea, such that with increasing duration, theadjusted value of EPAP exponentially approaches a value that is greaterthan a maximum EPAP value, to improve the ability of the EPAP to splintthe airway during ventilation.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that reduce the number offalse negatives in hypopnea detection by detecting hypopnea dependenton: an extent to which pressure support being delivered to the patientis large; and an extent to which a measure of absolute value of airflowof the patient is small compared to a target absolute airflow.

These two most recently described aspects may be particularlyadvantageous when used in combination.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that increase an EPAPvalue according to a computed measure of M-shaped inspiratory flowlimitation, such that the amount of increase is dependent on a ratio ofbreathwise ventilation to typical recent ventilation, so as to reducethe effect of “behavioural” breaths on the EPAP value.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that compute a measure ofM-shaped inspiratory flow limitation of a patient based on a version ofan inspiratory flow waveform that is symmetrised around a location of anotch in an inspiratory flow waveform.

These two most recently described aspects may be particularlyadvantageous when used in combination.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that increase an EPAPvalue according to a computed measure of reverse-chairness ofinspiratory flow limitation, such that the amount of increase depends onthe consistency of reverse-chairness between current and precedingbreaths, so as to reduce the adverse consequences of EPAP increase.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that compute a measure ofreverse chairness of inspiratory flow limitation of a patient dependenton the extent of recent uncompensated leak in the delivery of airflow tothe patient.

These two most recently described aspects may be particularlyadvantageous when used in combination.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that increase an EPAPvalue according to a computed measure of inspiratory snore, in theabsence of expiratory snore.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that compute a measure ofinspiratory snore as a mean over an inspiratory portion of a currentbreath of a difference between the output of a snore filter on aninstantaneous interface pressure and a threshold that is dependent onthe instantaneous interface pressure.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that compute a measure ofexpiratory snore using joint thresholds on duration and intensity of theoutput of a snore filter on an instantaneous interface pressure duringan expiratory portion of a current breath.

These three most recently described aspects may be particularlyadvantageous when used in combination, so as to reduce EPAP increasesdue to “spurious snore”.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that estimate a phase ofa current breathing cycle of a patient, such that a weight given to astandard rate of change in the phase estimate is dependent on an extentto which the patient has recently been achieving ventilation at or abovea target ventilation, so as to improve tolerance of lower respiratoryrates and short-term variations in the respiratory rate.

This most recently described aspect may be used in combination with anyof the previously described aspects or combinations thereof.

Another aspect of one form of the present technology comprises apparatusor methods for treating a respiratory disorder that deliver pressuresupport to a patient at a value that is a combination of: a value ofpressure support that is sufficient to increase instantaneousventilation to a target ventilation; and a value of pressure supportthat is sufficient to increase gross alveolar ventilation to a targetgross alveolar ventilation, so as to treat patients with periodicbreathing and respiratory insufficiency.

This most recently described aspect may be used in combination with anyof the previously described aspects or combinations thereof.

Other aspects of the present technology comprise computer readablestorage media having recorded thereon computer program code that isconfigured to cause a processor to carry out methods according to theabove described aspects.

Of course, portions of the aspects may form sub-aspects of the presenttechnology. Also, various ones of the sub-aspects and/or aspects may becombined in various manners and also constitute additional aspects orsub-aspects of the present technology.

Other features of the technology will be apparent from consideration ofthe information contained in the following detailed description,abstract, drawings and claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present technology is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings, in whichlike reference numerals refer to similar elements including:

Treatment Systems

FIG. 1 a shows a system in accordance with the present technology. Apatient 1000 wearing a patient interface 3000, receives a supply of airat positive pressure from a PAP device 4000. Air from the PAP device ishumidified in a humidifier 5000, and passes along an air circuit 4170 tothe patient 1000.

Respiratory System

FIG. 2 a shows an overview of a human respiratory system including thenasal and oral cavities, the larynx, vocal folds, oesophagus, trachea,bronchus, lung, alveolar sacs, heart and diaphragm.

FIG. 2 b shows a view of a human upper airway including the nasalcavity, nasal bone, lateral nasal cartilage, greater alar cartilage,nostril, lip superior, lip inferior, larynx, hard palate; soft palate,oropharynx, tongue, epiglottis, vocal folds, oesophagus and trachea.

Patient Interface

FIG. 3 a shows a patient interface in accordance with one form of thepresent technology.

Pap Device

FIG. 4 a shows a PAP device in accordance with one form of the presenttechnology.

FIG. 4 b shows a schematic diagram of the pneumatic circuit of a PAPdevice of FIG. 4 a. The directions of upstream and downstream areindicated.

FIG. 4 c shows a schematic diagram of the electrical components of thePAP device of FIG. 4 a.

FIG. 4 d shows a schematic diagram of the algorithms implemented in thePAP device of FIG. 4 a. In this figure, arrows with solid lines indicatean actual flow of information, for example via an electronic signal.

Humidifier

FIG. 5 a shows a humidifier in accordance with one aspect of the presenttechnology.

Breathing Waveforms

FIG. 6 a shows a model typical breath waveform of a person whilesleeping. The horizontal axis is time, and the vertical axis isrespiratory flow. While the parameter values may vary, a typical breathmay have the following approximate values: tidal volume, Vt, 0.5 L,inhalation time, Ti, 1.6 s, peak inspiratory flow, Qpeak, 0.4 L/s,exhalation time, Te, 2.4 s, peak expiratory flow, Qpeak, −0.5 L/s. Thetotal duration of the breath, Ttot, is about 4 s. The person typicallybreathes at a breathing rate of about 15 breaths per minute (BPM), with.Ventilation, Vent, about 7.5 L/minute. A typical duty cycle, the ratioof Ti to Ttot is about 40%.

FIG. 6 b shows a patient during non-REM sleep breathing normally over aperiod of about ninety seconds, with about 34 breaths, being treatedwith Automatic PAP, and the mask pressure being about 11 cm H₂O. The topchannel shows oximetry (SpO₂), the scale has a range of saturation from90 to 99% in the vertical direction. The patient maintained a saturationof about 95% throughout the period shown. The second channel showsquantitative respiratory airflow, and the scale ranges from −1 to +1 LPSin a vertical direction, and with inspiration positive. Thoracic andabdominal movement are shown in the third and fourth channels.

FIG. 6 c shows polysomnography of a patient before treatment. There areeleven signal channels from top to bottom with a 6 minute horizontalspan. The top two channels both are EEG (electroencephalogram) fromdifferent scalp locations. Periodic spikes in second represent corticalarousal and related activity. The third channel down is submental EMG(electromyogram). Increasing activity around time of arousals representgenioglossus recruitment. The fourth & fifth channels are EOG(electro-oculogram). The sixth channel is an electrocardiogram. Theseventh channel shows pulse oximetry (SpO₂) with repetitivedesaturations to below 70% from about 90%. The eighth channel isrespiratory airflow using nasal cannula connected to differentialpressure transducer. Repetitive apneas of 25 to 35 seconds alternatingwith 10 to 15 second bursts of recovery breathing coinciding with EEGarousal and increased EMG activity. The ninth shows movement of chestand tenth shows movement of abdomen. The abdomen shows a crescendo ofmovement over the length of the apnea leading to the arousal. Bothbecome untidy during the arousal due to gross body movement duringrecovery hyperpnea. The apneas are therefore obstructive, and thecondition is severe. The lowest channel is posture, and in this exampleit does not show change.

FIG. 6 d shows patient flow data where the patient is experiencing aseries of total obstructive apneas. The duration of the recording isapproximately 160 seconds. Flow ranges from about +1 L/s to about −1.5L/s. Each apnea lasts approximately 10-15 s.

FIG. 6 e shows a scaled inspiratory portion of a breath where thepatient is experiencing low frequency inspiratory snore.

FIG. 6 f shows a scaled inspiratory portion of a breath where thepatient is experiencing an example of ordinary or “mesa” flatnessinspiratory flow limitation.

FIG. 6 g shows a scaled inspiratory portion of a breath where thepatient is experiencing an example of “reverse chair” inspiratory flowlimitation.

FIG. 6 h shows a scaled inspiratory portion of a breath where thepatient is experiencing an example of “M-shaped” inspiratory flowlimitation.

FIG. 6 i illustrates an example of Cheyne-Stokes respiration. There arethree channels: oxygen saturation (SpO₂), a signal indicative of flow,and movement. The data span six minutes. The signal representative offlow was measured using a pressure sensor connected to nasal cannulae.The patient exhibits apneas of about 22 seconds and hyperpneas of about38 seconds. Higher frequency low amplitude oscillation during apnea iscardiogenic.

FIGS. 7 a to 7 q are flow charts illustrating the operation of thealgorithms of FIG. 4 d in one form of the PAP device of FIG. 4 a.

FIG. 8 illustrates an example “smooth and comfortable” treatmentpressure waveform as a function of phase in accordance with one form ofthe present technology.

DETAILED DESCRIPTION OF EXAMPLES OF THE TECHNOLOGY

Before the present technology is described in further detail, it is tobe understood that the technology is not limited to the particularexamples described herein, which may vary. It is also to be understoodthat the terminology used in this disclosure is for the purpose ofdescribing only the particular examples discussed herein, and is notintended to be limiting.

Treatment Systems

In one form, the present technology comprises apparatus for treating arespiratory disorder. The apparatus may comprise a flow generator orblower for supplying pressurised respiratory gas, such as air, to thepatient 1000 via an air delivery tube leading to a patient interface3000.

Therapy

In one form, the present technology comprises a method for treating arespiratory disorder comprising the step of applying positive pressureto the entrance of the airways of a patient 1000.

Nasal CPAP for OSA

In one form, the present technology comprises a method of treatingObstructive Sleep Apnea in a patient by applying nasal continuouspositive airway pressure to the patient.

In certain embodiments of the present technology, a supply of air atpositive pressure is provided to the nasal passages of the patient viaone or both nares.

Patient Interface 3000

A non-invasive patient interface 3000 in accordance with one aspect ofthe present technology comprises the following functional aspects: aseal-forming structure 3100, a plenum chamber 3200, a positioning andstabilising structure 3300 and a connection port 3600 for connection toair circuit 4170. In some forms a functional aspect may be provided byone or more physical components. In some forms, one physical componentmay provide one or more functional aspects. In use the seal-formingstructure 3100 is arranged to surround an entrance to the airways of,the patient so as to facilitate the supply of air at positive pressureto the airways.

Pap Device 4000

A PAP device 4000 in accordance with one aspect of the presenttechnology comprises mechanical and pneumatic components 4100,electrical components 4200 and is programmed to execute one or morealgorithms 4300. The PAP device has an external housing 4010 formed intwo parts, an upper portion 4012 of the external housing 4010, and alower portion 4014 of the external housing 4010. In alternative forms,the external housing 4010 may include one or more panel(s) 4015. The PAPdevice 4000 comprises a chassis 4016 that supports one or more internalcomponents of the PAP device 4000. In one form a pneumatic block 4020 issupported by, or formed as part of the chassis 4016. The PAP device 4000may include a handle 4018.

The pneumatic path of the PAP device 4000 comprises an inlet air filter4112, an inlet muffler 4122, a controllable pressure device 4140 capableof supplying air at positive pressure (preferably a blower 4142), and anoutlet muffler 4124. One or more pressure sensors 4272 and flow sensors4274 are included in the pneumatic path.

The pneumatic block 4020 comprises a portion of the pneumatic path thatis located within the external housing 4010.

The PAP device 4000 has an electrical power supply 4210, one or moreinput devices 4220, a central controller 4230, a therapy devicecontroller 4240, a therapy device 4245, one or more protection circuits4250, memory 4260, transducers 4270, data communication interface 4280and one or more output devices 4290. Electrical components 4200 may bemounted on a single Printed Circuit Board Assembly (PCBA) 4202. In analternative form, the PAP device 4000 may include more than one PCBA4202.

The central controller 4230 of the PAP device 4000 is programmed toexecute one or more algorithm modules 4300, including in oneimplementation a pre-processing module 4310, a therapy engine module4320, a pressure control module 4330, and a fault condition module 4340.

In what follows, the PAP device 4000 is referred to interchangeably as aventilator.

PAP Device Mechanical & Pneumatic Components 4100 Air Filter(s) 4110

A PAP device in accordance with one form of the present technology mayinclude an air filter 4110, or a plurality of air filters 4110.

In one form, an inlet air filter 4112 is located at the beginning of thepneumatic path upstream of a blower 4142. See FIG. 4 b.

In one form, an outlet air filter 4114, for example an antibacterialfilter, is located between an outlet of the pneumatic block 4020 and apatient interface 3000. See FIG. 4 b.

Muffler(s) 4120

In one form of the present technology, an inlet muffler 4122 is locatedin the pneumatic path upstream of a blower 4142. See FIG. 4 b.

In one form of the present technology, an outlet muffler 4124 is locatedin the pneumatic path between the blower 4142 and a patient interface3000. See FIG. 4 b.

Pressure Device 4140

In one form of the present technology, a pressure device 4140 forproducing a flow of air at positive pressure is a controllable blower4142. For example, the blower may include a brushless DC motor 4144 withone or more impellers housed in a volute. The blower is capable ofdelivering a supply of air, for example about 120 litres/minute, at apositive pressure in a range from about 4 cm H₂O to about 20 cm H₂O, orin other forms up to about 30 cm H₂O.

The pressure device 4140 is under the control of the therapy devicecontroller 4240.

Transducer(s) 4270

In one form of the present technology, one or more transducers 4270 arelocated upstream of the pressure device 4140. The one or moretransducers 4270 are constructed and arranged to measure properties ofthe air at that point in the pneumatic path.

In one form of the present technology, one or more transducers 4270 arelocated downstream of the pressure device 4140, and upstream of the aircircuit 4170. The one or more transducers 4270 are constructed andarranged to measure properties of the air at that point in the pneumaticpath.

In one form of the present technology, one or more transducers 4270 arelocated proximate to the patient interface 3000.

Anti-Spill Back Valve 4160

In one form of the present technology, an anti-spill back valve islocated between the humidifier 5000 and the pneumatic block 4020. Theanti-spill back valve is constructed and arranged to reduce the riskthat water will flow upstream from the humidifier 5000, for example tothe motor 4144.

Air Circuit 4170

An air circuit 4170 in accordance with an aspect of the presenttechnology is constructed and arranged to allow a flow of air orbreathable gasses between the pneumatic block 4020 and the patientinterface 3000.

Oxygen Delivery 4180

In one form of the present technology, supplemental oxygen 4180 isdelivered to a point in the pneumatic path.

In one form of the present technology, supplemental oxygen 4180 isdelivered upstream of the pneumatic block 4020.

In one form of the present technology, supplemental oxygen 4180 isdelivered to the air circuit 4170.

In one form of the present technology, supplemental oxygen 4180 isdelivered to the patient interface 3000.

PAP Device Electrical Components 4200 Power Supply 4210

In one form of the present technology power supply 4210 is internal ofthe external housing 4010 of the PAP device 4000. In another form of thepresent technology, power supply 4210 is external of the externalhousing 4010 of the PAP device 4000.

In one form of the present technology power supply 4210 provideselectrical power to the PAP device 4000 only. In another form of thepresent technology, power supply 4210 provides electrical power to bothPAP device 4000 and humidifier 5000.

Input Devices 4220

In one form of the present technology, a PAP device 4000 includes one ormore input devices 4220 in the form of buttons, switches or dials toallow a person to interact with the device. The buttons, switches ordials may be physical devices, or software devices accessible via atouch screen. The buttons, switches or dials may, in one form, bephysically connected to the external housing 4010, or may, in anotherform, be in wireless communication with a receiver that is in electricalconnection to the central controller 4230.

In one form the input device 4220 may be constructed and arranged toallow a person to select a value and/or a menu option.

Central Controller 4230

In one form of the present technology, the central controller 4230 is aprocessor suitable to control a PAP device 4000 such as an x86 INTELprocessor.

A processor 4230 suitable to control a PAP device 4000 in accordancewith another form of the present technology includes a processor basedon ARM Cortex-M processor from ARM Holdings. For example, an STM32series microcontroller from ST MICROELECTRONICS may be used.

Another processor 4230 suitable to control a PAP device 4000 inaccordance with a further alternative form of the present technologyincludes a member selected from the family ARM9-based 32-bit RISC CPUs.For example, an STR9 series microcontroller from ST MICROELECTRONICS maybe used.

In certain alternative forms of the present technology, a 16-bit RISCCPU may be used as the processor 4230 for the PAP device 4000. Forexample a processor from the MSP430 family of microcontrollers,manufactured by TEXAS INSTRUMENTS, may be used.

The processor 4230 is configured to receive input signal(s) from one ormore transducers 4270, and one or more input devices 4220.

The processor 4230 is configured to provide output signal(s) to one ormore of an output device 4290, a therapy device controller 4240, a datacommunication interface 4280 and humidifier controller 5250.

The processor 4230, or multiple such processors, may be configured toimplement the one or more methodologies described herein such as one ormore algorithms 4300 expressed as computer programs stored in a computerreadable storage medium, such as memory 4260. In some cases, aspreviously discussed, such processor(s) may be integrated with a PAPdevice 4000. However, in some devices the processor(s) may beimplemented discretely from the flow generation components of the PAPdevice, such as for purpose of performing any of the methodologiesdescribed herein without directly controlling delivery of a respiratorytreatment. For example, such a processor may perform any of themethodologies described herein for purposes of determining controlsettings for a ventilator or other respiratory related events byanalysis of stored data such as from any of the sensors describedherein.

Clock 4232

Preferably PAP device 4000 includes a clock 4232 that is connected toprocessor 4230.

Therapy Device Controller 4240

In one form of the present technology, therapy device controller 4240 isa pressure control module 4330 that forms part of the algorithms 4300executed by the processor 4230.

Ins one form of the present technology, therapy device controller 4240is a dedicated motor control integrated circuit. For example, in oneform a MC33035 brushless DC motor controller, manufactured by ONSEMI isused.

Protection Circuits 4250

Preferably a PAP device 4000 in accordance with the present technologycomprises one or more protection circuits 4250.

One form of protection circuit 4250 in accordance with the presenttechnology is an electrical protection circuit.

One form of protection circuit 4250 in accordance with the presenttechnology is a temperature or pressure safety circuit.

Memory 4260

In accordance with one form of the present technology the PAP device4000 includes memory 4260, preferably non-volatile memory. In someforms, memory 4260 may include battery powered static RAM. In someforms, memory 4260 may include volatile RAM.

Preferably memory 4260 is located on PCBA 4202. Memory 4260 may be inthe form of EEPROM, or NAND flash.

Additionally or alternatively, PAP device 4000 includes removable formof memory 4260, for example a memory card made in accordance with theSecure Digital (SD) standard.

In one form of the present technology, the memory 4260 acts as acomputer readable storage medium on which is stored computer programinstructions expressing the one or more methodologies described herein,such as the one or more algorithms 4300.

Transducers 4270

Transducers may be internal of the device, or external of the PAPdevice. External transducers may be located for example on or form partof the air delivery circuit, e.g. the patient interface. Externaltransducers may be in the form of non-contact sensors such as a Dopplerradar movement sensor that transmit or transfer data to the PAP device.

Flow 4274

A flow transducer 4274 in accordance with the present technology may bebased on a differential pressure transducer, for example, an SDP600Series differential pressure transducer from SENSIRION. The differentialpressure transducer is in fluid communication with the pneumaticcircuit, with one of each of the pressure transducers connected torespective first and second points in a flow restricting element.

In use, a signal or total flow Qt signal, from the flow transducer 4274,is received by the processor 4230. However, other sensors for producingsuch a flow signal or estimating flow may be implemented. For example, amass flow sensor, such as a hot wire mass flow sensor, may beimplemented to generate a flow signal in some embodiments. Optionally,flow may be estimated from one or more signals of other sensorsdescribed here, such as in accordance with any of the methodologiesdescribed in a U.S. patent application Ser. No. 12/192,247, thedisclosure of which is incorporated herein by reference.

Pressure 4272

A pressure transducer 4272 in accordance with the present technology islocated in fluid communication with the pneumatic circuit. An example ofa suitable pressure transducer is a sensor from the HONEYWELL ASDXseries. An alternative suitable pressure transducer is a sensor from theNPA Series from GENERAL ELECTRIC.

In use, a signal from the pressure transducer 4272, is received by theprocessor 4230. In one form, the signal from the pressure transducer4272 is filtered prior to being received by the processor 4230.

Motor Speed 4276

In one form of the present technology a motor speed signal 4276 isgenerated. A motor speed signal 4276 is preferably provided by therapydevice controller 4240. Motor speed may, for example, be generated by aspeed sensor, such as a Hall effect sensor.

Data Communication Systems 4280

In one preferred form of the present technology, a data communicationinterface 4280 is provided, and is connected to processor 4230. Datacommunication interface 4280 is preferably connectable to remoteexternal communication network 4282. Data communication interface 4280is preferably connectable to local external communication network 4284.Preferably remote external communication network 4282 is connectable toremote external device 4286. Preferably local external communicationnetwork 4284 is connectable to local external device 4288.

In one form, data communication interface 4280 is part of processor4230. In another form, data communication interface 4280 is anintegrated circuit that is separate from processor 4230.

In one form, remote external communication network 4282 is the Internet.The data communication interface 4280 may use wired communication (e.g.via Ethernet, or optical fibre) or a wireless protocol to connect to theInternet.

In one form, local external communication network 4284 utilises one ormore communication standards, such as Bluetooth, or a consumer infraredprotocol.

In one form, remote external device 4286 is one or more computers, forexample a cluster of networked computers. In one form, remote externaldevice 4286 may be virtual computers, rather than physical computers. Ineither case, such remote external device 4286 may be accessible to anappropriately authorised person such as a clinician.

Preferably local external device 4288 is a personal computer, mobilephone, tablet or remote control.

Output Devices Including Optional Display, Alarms 4290

An output device 4290 in accordance with the present technology may takethe form of one or more of a visual, audio and haptic unit. A visualdisplay may be a Liquid Crystal Display (LCD) or Light Emitting Diode(LED) display.

Display Driver 4292

A display driver 4292 receives as an input the characters, symbols, orimages intended for display on the display 4294, and converts them tocommands that cause the display 4294 to display those characters,symbols, or images.

Display 4294

A display 4294 is configured to visually display characters, symbols, orimages in response to commands received from the display driver 4292.For example, the display 4294 may be an eight-segment display, in whichcase the display driver 4292 converts each character or symbol, such asthe figure “0”, to eight logical signals indicating whether the eightrespective segments are to be activated to display a particularcharacter or symbol.

PAP Device Algorithms 4300 Pre-Processing Module 4310

A pre-processing module 4310 in accordance with the present technologyreceives as an input raw data from a transducer, for example a flow orpressure transducer, and preferably performs one or more process stepsto calculate one or more output values that will be used as an input toanother module, for example a therapy engine module 4320.

In one form of the present technology, the output values include theinterface or mask pressure Pm, the respiratory flow Qr, and the leakflow Ql.

In various forms of the present technology, the pre-processing module4310 comprises one or more of the following algorithms: pressurecompensation 4312, vent flow 4314, leak flow 4316, respiratory flow4318, and jamming detection4319.

Pressure Compensation 4312

In one form of the present technology, a pressure compensation algorithm4312 receives as an input a signal indicative of the pressure in thepneumatic path proximal to an outlet of the pneumatic block. Thepressure compensation algorithm 4312 estimates the pressure drop in theair circuit 4170 and provides as an output an estimated pressure, Pm, inthe patient interface 3000.

Vent Flow 4314

In one form of the present technology, a vent flow calculation algorithm4314 receives as an input an estimated pressure, Pm, in the patientinterface 3000 and estimates a vent flow of air, Qv, from a vent 3400 ina patient interface 3000.

Leak Flow 4316

In one form of the present technology, a leak flow algorithm 4316receives as an input a total flow, Qt, and a vent flow Qv, and providesas an output a leak flow Ql by calculating an average of Qt−Qv over aperiod sufficiently long to include several breathing cycles, e.g. about10 seconds.

In one form, the leak flow algorithm 4316 receives as an input a totalflow, Qt, a vent flow Qv, and an estimated pressure, Pm, in the patientinterface 3000, and provides as an output a leak flow Ql by calculatinga leak conductance, and determining a leak flow Ql to be a function ofleak conductance and interface pressure, Pm. In one implementation, leakconductance is calculated as the quotient of low pass filtered non-ventflow Qt−Qv, and low pass filtered square root of mask pressure Pm, wherethe low pass filter time constant has a value sufficiently long toinclude several breathing cycles, e.g. about 10 seconds.

Respiratory Flow 4318

In one form of the present technology, a respiratory flow algorithm 4318receives as an input a total flow, Qt, a vent flow, Qv, and a leak flow,Ql, and estimates a respiratory flow to the patient, Qr, by subtractingthe vent flow Qv and the leak flow Ql from the total flow Qt.

Jamming Detection 4319

When the leak has recently changed and the leak flow algorithm 4316 hasnot fully compensated for the change, a state designated as “jamming”exists, which may be determined according to the methods described inU.S. Pat. No. 6,532,957, U.S. Pat. No. 6,810,876 or, U.S. PatentApplication Publication No. 2010/0101574 A1, the disclosures of whichare incorporated herein by reference. In the jamming state, therespiratory flow baseline is usually incorrect to some degree, whichdistorts flow shapes and affects the detection of flow limitation. Forexample, if the respiratory flow baseline is above the true level,respiratory flow in late expiration will be positive and thus be takenas early inspiratory flow; if this is expiratory pause flow, the truestart of inspiration may be taken as the increase after the flat portionof a reverse chair waveform. Hence a fuzzy truth variable,RecentJamming, which represents the extent to which jamming, i.e.uncompensated leak, has recently existed, is calculated by the jammingalgorithm 4319.

In the algorithm 4319, an instantaneous jamming fuzzy truth variable Jis calculated as the fuzzy extent to which the absolute magnitude of therespiratory flow Qr has been large for longer than expected. The fuzzyextent A₁ to which the airflow has been positive for longer thanexpected is calculated from the time t_(Z1) since the lastpositive-going zero crossing of the respiratory flow Qr, and theinspiratory duration Ti, using the following fuzzy membership function:

A ₁=FuzzyMember(t _(Z1) ,Ti,0,2*Ti,1)  (1)

The fuzzy extent B₁ to which the airflow is large and positive iscalculated from the respiratory flow Qr using following the fuzzymembership function:

B ₁=FuzzyMember(Qr,0,0,0.5,1)  (2)

The fuzzy extent I₁ to which the leak has suddenly increased iscalculated as the fuzzy “and” of the fuzzy truth variables A₁ and B₁.

Precisely symmetrical calculations are performed for expiration,deriving I_(E) as the fuzzy extent to which the leak has suddenlydecreased. The fuzzy extent A_(E) to which the airflow has been negativefor longer than expected is calculated from the time t_(ZE) since thelast negative-going zero crossing of the respiratory flow Qr, and theexpiratory duration Te, using the fuzzy membership function in equation(1). The fuzzy extent B_(E) to which the airflow is large and negativeis calculated from the negative of the respiratory flow Qr using thefuzzy membership function in equation (2), and I_(E) is calculated asthe fuzzy “and” of the fuzzy truth variables A_(E) and B_(E). Theinstantaneous jamming index J is calculated as the fuzzy “or” of thefuzzy truth variables I₁ and I_(E).

If the instantaneous jamming value J is larger than the recent peakvalue of J, then RecentJamming is set to the instantaneous jamming valueJ. Otherwise, RecentJamming is set to the instantaneous jamming value J,low pass filtered with a time constant of 10 seconds.

Therapy Engine Module 4320

In one form of the present technology, a therapy engine module 4320receives as inputs one or more of a pressure, Pm, in a patient interface3000, a respiratory flow of air to a patient, Qr, a leak flow, Ql, ajamming fuzzy truth variable, RecentJamming, and provides as an outputone or more therapy parameters.

In one form of the present technology, the therapy parameter is the CPAPtreatment pressure Pt.

In another form of the present technology, the therapy parameters arethe EPAP, a waveform value, and a level of pressure support.

In another form of the present technology, the therapy parameters arethe EPAP, a waveform value, a target ventilation, and an instantaneousventilation.

In various forms of the present technology, the therapy engine module4320 comprises one or more of the following algorithms: phasedetermination 4321, waveform determination 4322, ventilationdetermination 4323, flow limitation determination 4324, apnea/hypopneadetermination 4325, snore determination 4326, EPAP determination 4327,target ventilation determination 4328, and therapy parameterdetermination 4329.

In FIGS. 7 a to 7 q that illustrate the operation of the therapy enginemodule 4320, solid connecting lines indicate control flow, while dashedconnecting lines indicate data flow.

Phase Determination 4321

In one form of the present technology, a phase determination algorithm4321 receives as an input a signal indicative of respiratory flow, Qr,and provides an estimate Φ of the phase of a breathing cycle of thepatient 1000. The rate of change of phase is indicative of therespiratory rate.

In one form, the phase estimate Φ is a discrete variable with values ofeither inhalation or exhalation. In one form, the phase estimate Φ isdetermined to have a discrete value of inhalation when a respiratoryflow Qr has a positive value that exceeds a positive threshold. In oneform, the phase estimate Φ is determined to have a discrete value ofexhalation when a respiratory flow Qr has a negative value that is morenegative than a negative threshold.

In one form, the phase estimate Φ is a discrete variable with values ofone of inhalation, mid-inspiratory pause, and exhalation.

In one form, the phase estimate Φ is a continuous variable, for examplevarying from 0 to 1, or 0 to 2π, or 0° to 360°. A phase estimate Φ equalto 0.5 (or π or 180°) occurs at the transition from inspiration toexpiration.

In one form of the present technology, the phase determination algorithm4321 uses fuzzy phase estimation as described in U.S. Pat. No.6,532,957, the disclosure of which is incorporated herein by reference,with a number of adjustments. In general, the philosophy behind theadjustments is to be more tolerant of lower respiratory rates andshort-term variations in respiratory rate. The general phase rules aregiven more weight at lower levels of ventilation than previously,improving patient synchronisation. This more than compensates for themild reduction in prescriptiveness of the ventilator with respect tomaintenance of target ventilation and respiratory rate in the very shortterm, over one or two breaths.

The “standard rate” of respiration, which corresponds to a kind ofbackup rate in conventional ventilators, is given a certain weight thatdepends on the degree of “trouble”, a fuzzy logical variable dependenton the degree of jamming, the degree of hypopnea, and the extent towhich leak is large. Even in the absence of “trouble”, the standard rateis given significant weight. This tends to cause the ventilator's breathrate to be pulled towards the standard rate, and tends to causedyssynchrony when the patient's respiratory rate is lower than thestandard rate, which in one implementation is set at 15 breaths/minute.Awake patients who want to breathe at lower rates, particularly duringthe sleep onset phase, can feel pushed along by this. A common reactionis to fight the ventilator, resulting in hypoventilation (from theperspective of the ventilator), which further increases the weight givento the standard rate, and higher pressure support.

To counteract this effect, and thereby increase patient comfort, in oneform of the present technology, the weight given to the standard rateindependent of “trouble” by the algorithm 4321 depends on the minimumpressure support (minimum swing) and the amount of pressure supportabove the minimum pressure support (“servo swing”), which is determinedby the algorithm 4329. Broadly, the idea is that low servo swing levelsindicate that the patient has recently been achieving ventilation at orabove the target ventilation, and so should be allowed to breathe atwhatever rate the patient chooses. Progressively higher servo swinglevels progressively indicate that this is less the case. The actualfuzzy membership calculation is performed using the current swing (thesum of the minimum and servo swings), using boundaries (SLow and SHigh)which depend on the minimum swing. The fuzzy truth variableSwingIsLargeForStdRate is the fuzzy extent to which the swing is large,for the purposes of determining the weight to be given to the standardrate (in fact the weight to be given to the standard rates of change ofphase for inspiration and expiration, since in general these aredifferent) independent of “trouble”.

FIG. 7 a is a flow chart illustrating a method 7100 that may be used toimplement algorithm 4321 in one form of the present technology. Themethod 7100 begins at step 7110 by computing the lower boundary SLow asa generally increasing function of the minimum swing value MinSwing. Inone implementation, SLow is computed as follows:

SLow=Interp(MinSwing,0,3,6,6,8,8)  (3)

At step 7120, the upper boundary SHigh is computed as a generallyincreasing function of the minimum swing, such that SHigh is alwaysgreater than or equal to the lower boundary SLow. In one implementation,SHigh is computed as follows:

SHigh=Interp(MinSwing,0,6,6,8,8,8)  (4)

At step 7130, the method 7100 computes the swing as the sum of theminimum swing and the current servo swing (pressure support aboveminimum). The method 7100 then at step 7140 computes the fuzzy truthvariable SwingIsLargeForStdRate as follows: At or above some rather highlevel of minimum swing, which in one implementation is 8 cm H₂O (whichshould not really occur in a ventilator designed to treat periodicbreathing of central origin), SwingIsLargeForStdRate is set to fuzzytrue. Otherwise, SwingIsLargeForStdRate transitions from fuzzy false tofuzzy true as swing increases between the lower and upper boundariesSLow and SHigh:

SwingIsLargeForStdRate=FuzzyMember(Swing,SLow,0,SHigh,1)  (5)

Finally, at step 7150 the method 7100 estimates the phase in the mannerdescribed in U.S. Pat. No. 6,532,957, except that the weight to thestandard breath rate, independent of “trouble”, is set to the computed,value of the fuzzy truth variable SwingIsLargeForStdRate.

The effect of the fuzzy truth function defined by equations (3), (4),and (5) is that both SLow and SHigh rise progressively as MinSwingincreases, and that the transition region, between SLow and SHigh, getsprogressively narrower as MinSwing increases, particularly as MinSwingexceeds 6, narrowing to zero at MinSwing=8, at which point SLow andSHigh both also equal 8, so that any value of swing equal to or abovethe minimum swing makes SwingIsLargeForStdRate fuzzy true.

An alternative implementation of the algorithm 4321 omits steps 7110 to7140 and instead directly computes a fuzzy truth variable indicating theextent to which the patient has recently been achieving ventilation ator above the target ventilation, rather than using low servo swing as anindication of this extent. Step 7150 estimates the phase as describedabove, giving weight to the standard breath rate, in the absence of“trouble”, equal to the value of the computed fuzzy truth variable

Waveform Determination 4322

In one form of the present technology, a control module 4330 controls atherapy device 4245 to provide positive airway pressure according to apredetermined waveform of pressure vs phase.

In one form of the present technology a waveform determination algorithm4322 receives as an input a value Φ indicative of the phase of thecurrent breathing cycle of the patient, and provides as an output awaveform value n)) in the range [0, 1].

In one form, the waveform is a square wave, having a value of 1 forearly values of phase corresponding to inspiration, and a value of 0 forlater values of phase corresponding to expiration. In other forms, thewaveform is a more “smooth and comfortable” waveform with a gradual riseto 1 for early values of phase, and a gradual fall to 0 for later valuesof phase. FIG. 8 illustrates an exemplary “smooth and comfortable”waveform Π(Φ), which rises to 1 as the phase increases from 0 to 0.5during inspiration, and falls to 0 as the phase increases from 0.5 to 1during expiration.

Ventilation Determination 4323

In one form of the present technology, a ventilation determinationalgorithm 4323 receives an input a respiratory flow Qr, and determines avalue of instantaneous patient ventilation, Vent.

In one form, the ventilation determination algorithm 4323 determines acurrent value of instantaneous patient ventilation, Vent, as the halfthe absolute value of respiratory flow, Qr.

Detection of Inspiratory Flow Limitation 4324

In one form of the present technology, a processor executes one or morealgorithms 4324 for the detection of inspiratory flow limitation.

In one form, the algorithm 4324 receives as an input a respiratory flowsignal Qr and computes one or more measures of the extent to which theinspiratory portion of the breath exhibits inspiratory flow limitation.

The algorithm 4324 computes measures of at least one of the followingthree types of inspiratory flow limitation: ordinary flatness, M-shape,and “reverse chairness” (see FIGS. 6 f, 6 h, and 6 g).

Flatness

Upper airway flow limitation not infrequently produces a respiratoryflow pattern during inspiration in which the airflow stabilises after arelatively short period of inspiration at a fairly stable level, beingthe level to which airflow is limited by a Starling valve phenomenonwell described in the literature, dropping typically late ininspiration. This period of fairly stable airflow appears “flat” in agraphical representation (see FIG. 6 f). An indication of flatness of aninspiratory waveform may be termed a flattening index (FI). Theflattening index of a square waveform is zero. A waveform which isconstant during its middle half at a value equal to the overall meanalso has a FI of zero; this can occur in practice when the initial riseabove the mean in the first quarter of the waveform balances the valuebelow the mean in the last quarter of the waveform. “High” values of theFI (e.g. >0.2) indicate mild or absent flow limitation.

FIG. 7 b is a flow chart illustrating a method 7200 that may be used tocompute a measure of flatness of inspiratory flow limitation as part ofthe algorithm 4324 in one form of the present technology. The method7200 starts at step 7210, which computes a flattening index from theinspiratory airflow waveform. In one implementation of step 7210, themean value of the inspiratory airflow waveform is calculated, the flowvalues are divided by the mean to produce a normalised waveform, and theRMS deviation of the middle half of the normalised waveform is theflattening index.

In some implementations of step 7210, the pointwise average of the mostrecent 5 breaths is carried out before the above FI calculation. Inother implementations, the FI is calculated on individual breaths andsome kind of filtering operation is performed on the recent FI values,such as taking the median of the last three FI values. In yet otherimplementations, there is no such filtering, such that the FI is derivedfrom only a single breath and a treatment response is directly based onthat single breath FI. The rationale for such single-breathimplementations is that during periodic breathing of predominantlycentral origin, such as CSR, the decline of respiratory effort and theonset of upper airway obstruction may be so rapid that there are onlyone or two flow-limited breaths before the onset of closed (i.e.obstructive) central apnea, or the flow-limited breaths may beintermingled with a variety of shapes not typically indicative of UAO,and it is desirable to respond rapidly to this evidence of flowlimitation.

Step 7220 calculates a fuzzy truth variable Flatness at the end of eachbreath that generally decreases as the flatness index for that breathincreases. In one implementation, Flatness is computed as follows:

Flatness=FuzzyMember(FI,0.05,1,0.15,0)  (6)

According to equation (6), Flatness is fuzzily true for any value of FIless than or equal to 0.05, because waveforms with FI≦0.05 appearequivalently flow limited to human assessment, the differences betweenthem mostly being due to noise or features unrelated to the degree offlow limitation.

M-Shape

M-shaped inspiratory flow waveforms, with tidal volumes or breathwiseventilation values not much greater than the typical recent values, areindicative of flow limitation. Such waveforms have a relatively rapidrise and fall and a dip or “notch” in flow approximately in the middle,the dip being due to flow limitation (see FIG. 6 h). At higher tidalvolumes or ventilation values, such waveforms are generally behavioural,i.e. microarousals during sleep, or sighs, and are not indicative offlow limitation. In a CPAP device, tidal volume or ventilation isgenerally decreased by M-shape, but a rapidly respondingservo-ventilator will tend to counteract such a fall in ventilation byincreasing the pressure support, so that a low ventilation level is notgenerally a helpful feature in deciding whether the waveform is actuallyflow-limited.

To detect M-shaped waveforms, the similarity of the inspiratory flowwaveform to a waveform which is broadly similar to an M shape isdetermined.

FIG. 7 c is a flow chart illustrating a method 7300 that may be used tocompute a measure of M-shaped inspiratory flow limitation as part of thealgorithm 4324 in one form of the present technology.

Since the notch may not be at the centre of the inspiratory flowwaveform, the method 7300 attempts to find the location of the notch,and then linearly time-distorts the waveform so that the notch is at thecentre of the waveform. To find the notch, the first step 7310 performsa modified convolution of the normalised inspiratory flow waveform f(t)(wherein the normalisation division by the mean) with a V-shaped kernelV(t) of length Ti/2, centred on zero, where Ti is the duration ofinspiration:

$\begin{matrix}{{V(t)} = {{8{\frac{t}{T_{i}}}} - 1}} & (7)\end{matrix}$

The modified convolution is based on separate convolutions with the leftand right halves of the kernel V(t). The left half convolution iscalculated as

$\begin{matrix}{{I_{L}(\tau)} = {\int_{\frac{- T_{i}}{4}}^{0}{{V(t)}{f( {t - \tau} )}{t}}}} & (8)\end{matrix}$

and the right half convolution as

$\begin{matrix}{{I_{R}(\tau)} = {\int_{0}^{\frac{T_{i}}{4}}{{V(t)}{f( {t - \tau} )}{t}}}} & (9)\end{matrix}$

The modified convolution I(τ) is computed as a combination of the leftand right half convolutions I_(L)(τ) and I_(R)(τ) such that if either ofthe left and right half convolutions is zero, the result is zero,regardless of the other quantity, and if both are 1, the result is 1.Thus constrained, the combination of the left and right halfconvolutions resembles a logical “and” function in some sense, hence isgiven the name “V-anded convolution”. In one implementation, thecombination is a modified geometric mean of the left and right halfconvolutions:

$\begin{matrix}{{I(\tau)} = \{ \begin{matrix}{\sqrt{{I_{L}(\tau)}{I_{R}(\tau)}},} & {{I_{L}(\tau)} > {0\mspace{14mu} {and}\mspace{14mu} {I_{R}(\tau)}} > 0} \\0 & {otherwise}\end{matrix} } & (10)\end{matrix}$

The above constraint provides a condition that the inspiratory flowwaveform to the left of the posited notch is generally increasingleftwards, and that to the right of the notch is generally increasingrightwards. This provides more specificity than simply summing the leftand right integrals. In the implementation given in equation (10), theintegrals of the product of the time-shifted normalised inspiratory flowwaveform with each half-V must be strictly positive, otherwise theV-anded convolution is zero. This prevents a variety of pathologies, forexample, when the part of the inspiratory flow to the left of the centreof the V does not actually increase leftwards, but the integral of theright half of the V waveform is so large that it overwhelms an actuallydecreasing left half.

The V-anded convolution is performed with the position of the centre ofthe kernel V(t) ranging from Ti/4 to 3 Ti/4, thus yielding results forthe central half of the inspiratory flow waveform.

Step 7320 finds the location at which the modified convolution I(τ)peaks, and if the height of this peak is greater than a threshold, anotch is deemed to exist at the location t_(notch) of the centre of thekernel V(t) at which this peak is located. In one implementation, thethreshold is set to 0.15.

If a notch is found by step 7320 (“Y”) at the location t_(notch), theinspiratory flow waveform f(t) is then, at step 7330, time distorted or“symmetrised” so that half the waveform is to the left of t_(notch) andhalf is to the right. This operation gives a time-distorted or“symmetrised” version G(t) of the flow waveform f(t):

$\begin{matrix}{{G(t)} = \{ \begin{matrix}{{f( {\frac{t}{( \frac{T}{2} )}t_{notch}} )},} & {t < \frac{T}{2}} \\{f( {t_{notch} + {( \frac{t - \frac{T}{2}}{\frac{T}{2}} )( {T - t_{notch}} )}} )} & {t \geq \frac{T}{2}}\end{matrix} } & (11)\end{matrix}$

If no notch is found at step 7320 (“N”), step 7335 sets G(t) to theinspiratory flow waveform f(t), since some waveforms that do not exhibita detectable notch may still have M-shaped flow limitation.

Define the inner product of two functions on some interval I in theusual way,

$\begin{matrix}{{\langle{f,g}\rangle}_{l} = {\int_{l}{{f(t)}{g(t)}{t}}}} & (12)\end{matrix}$

Define first and third sinusoidal harmonic functions of half-width Ti as

$\begin{matrix}{{{F_{1}(t)} = {\sin ( {\pi \frac{t}{Ti}} )}}{and}} & (13) \\{{F_{3}(t)} = {\sin ( {3\pi \frac{t}{Ti}} )}} & (14)\end{matrix}$

These two harmonic functions are orthogonal on [0, Ti]. For t in [0,Ti], F₃(t) is broadly similar to an M-shaped inspiratory waveform, andF₁(t) is broadly similar to a normal inspiratory waveform. Hence theextent to which the symmetrised waveform G(t) resembles F₃(t) is anindicator of how much the waveform resembles an M. Step 7340 calculatesthis extent. In one implementation, step 7340 calculates the extent asthe ratio M3Ratio of the power in the third harmonic of the symmetrisedwaveform G(t) to the sum Of the power in the first and third harmonics,where it is understood that if the inner product operator has nosubscript, the interval is the inspiratory interval [0,Ti]:

$\begin{matrix}{{M\; 3{Ratio}} = \frac{{\langle{F_{3},G}\rangle}^{2}}{{\langle{F_{1},G}\rangle}^{2} + {\langle{F_{3},G}\rangle}^{2}}} & (15)\end{matrix}$

When M3Ratio is large, the inspiratory flow waveform typically resemblesan M. But M3Ratio can also be large if the waveform is very asymmetric,with a much higher mean flow in either the first or second half of thewaveform than in the other half. To exclude this possibility, step 7340also calculates a measure Symm of the symmetry of the inspiratory flowwaveform f(t) about the notch location. In one implementation, step 7340calculates the third harmonic components of the first and second halvesof the symmetrised waveform G(t):

$\begin{matrix}{M_{3L} = {\langle{F_{3},G}\rangle}_{\lbrack{0,\frac{Ti}{2}}\rbrack}} & (16) \\{M_{3R} = {\langle{F_{3},G}\rangle}_{\lbrack{\frac{Ti}{2},{Ti}}\rbrack}} & (17)\end{matrix}$

Step 7340 then calculates the measure Symm as the ratio of the lesser ofthese components to the sum of their absolute values:

$\begin{matrix}{{Symm} = \frac{\min ( {M_{3L},M_{3R}} )}{{M_{3L}} + {M_{3R}}}} & (18)\end{matrix}$

Step 7350 then tests whether the measure Symm is less than a lowthreshold, set in one implementation to 0.3. If so (“Y”), theinspiratory flow waveform is deemed not to be symmetrically M-shaped,and a quantity M3RatioSym, which is a measure of the extent to which theinspiratory flow waveform is symmetrically M-shaped, is set equal tozero at step 7360. Otherwise (“N”), M3RatioSym is set equal to M3Ratioat step 7370.

Reverse chairness

In some patients with partial upper airway obstruction, the flowwaveform increases somewhat at the start of inspiration, staysapproximately steady, then later in inspiration rises significantly, tolevels suggesting an absence of obstruction, then declines to zero in afairly normal fashion towards the end of inspiration (see FIG. 6 g).This somewhat resembles a chair seen side-on, with the back of the chairat the end of inspiration; this is termed “reverse chairness”, becausethe typical obstructive sleep apnea flow-limited waveform resembles thisshape, but with the back of the chair near the start of inspiration. Inthe presence of significant pressure support, particularly with a“smooth and comfortable” pressure waveform such as illustrated in FIG.8, reverse chairness is thought to be due to an initial state of partialobstruction, with the rising pressure opening the upper airway duringinspiration, so that in the latter part of inspiration the airway issubstantially unobstructed. It has been observed that if this phenomenonis untreated, and the EPAP is progressively lowered, total upper airwayobstruction may result. Hence it is desirable to detect reversechairness and raise EPAP in response to it.

Various non-obstructive behavioural waveforms, most notably amicroarousal during sleep, or a sigh, may produce a waveform exhibitingreverse chairness, and so in the present technology measures are takento attempt not to respond to these non-obstructive causes of reversechairness.

FIG. 7 d is a flow chart illustrating a method 7400 that may be used tocompute a measure of reverse chairness of inspiratory flow limitation aspart of the algorithm 4324 in one form of the present technology.

The method 7400 starts at step 7410, at which a smoothed derivative ofthe inspiratory flow waveform is calculated. In one implementation, step7410 convolves the inspiratory flow waveform with the first derivativeof a Gaussian function with standard deviation 0.1 seconds. In otherimplementations of step 7410, various other means with similar frequencyresponse characteristics, such as a suitable low-pass filter followed bydifferentiation, are used. For detection of a shape characteristic suchas reverse chairness, it is desirable that a largely scale-independentderivative be used, and so in one implementation the smoothed derivative(in litres/sec/sec) is normalised at step 7415 by TypVent/9, whereTypVent is a measure of typical recent ventilation (in litres/min)(e.g., calculated as described below with reference to FIG. 7 o), givinga normalised derivative with the units sec⁻¹.

Step 7420 then performs shape recognition using a state machine withthree states, corresponding to the initial rise, the approximately flatregion, and the further rise. Starting at the beginning of theinspiratory flow waveform, in state “LookingForInitialPositive”, thenormalised derivative is traversed until it is found to be at least 0.3,whereupon the state transitions to “LookingForLevel”. In this state, asearch is performed, starting from the current position in thenormalised derivative, for a normalised derivative value less than 0.05,whereupon the state transitions to “LookingForPositive”. The minimum andmaximum normalised derivatives from this location onward arecontinuously updated as the search again proceeds, this time looking fora location at which the maximum normalised derivative has surpassed0.15, and the normalised derivative is at least 0.05 less than themaximum normalised derivative. The idea of the latter criterion is toprovide some hysteresis.

It is clear that as the search proceeds, the maximum normalisedderivative can increase, and for what follows it is desirable that amoderately large value is found. Without the criterion that thenormalised derivative has decreased moderately from its maximum, thesearch may terminate rather quickly. Step 7420 returns the differencebetween the maximum and minimum normalised derivatives, calledDerivativeRange. If the third state is never reached, DerivativeRange isreturned as zero. Step 7425 then tests whether DerivativeRange isgreater than some low threshold, equal to 0.2 in one implementation. Ifso (“Y”), the waveform is provisionally deemed reverse-chair-shaped, thelocation tmin of the minimum derivative is recorded, and the method 7400proceeds to step 7430. Otherwise (“N”), the method 7400 at step 7495sets a fuzzy truth variable ReverseChairnessCurrent to 0, and concludes.

Step 7430 computes a variable LateProportion, which is the proportion ofthe inspiratory tidal volume in the latter half of the inspiration,ignoring the first and last 10% of the inspiration by time:

$\begin{matrix}{{LateProportion} = \frac{\int_{0.5{Ti}}^{0.9{Ti}}{{{f(t)}}{t}}}{\int_{0.1{Ti}}^{0.9{Ti}}{{(t)}{t}}}} & (19)\end{matrix}$

Step 7430 also calculates a variable EarlyProportion, the proportion ofthe inspiratory tidal volume which occurs before the minimum derivativelocation tmin:

$\begin{matrix}{{EarlyProportion} = \frac{\int_{0}^{tmin}{{{f(t)}}{t}}}{\int_{0}^{Ti}{{{f(t)}}{t}}}} & (20)\end{matrix}$

Note the denominator of equation (20) is the inspiratory tidal volume, aquantity that is used later in the method 7400.

Step 7435 compares EarlyProportion to a low threshold, equal to 0.1 inone implementation. If EarlyProportion is less than the threshold (“Y”),the waveform is deemed not reverse-chair shaped, and the method 7400proceeds to step 7495 as described above. Otherwise (“N”), at step 7440the typical recent tidal volume (in litres per breath) is calculated. Inone implementation, step 7440 divides the typical recent ventilation inlitres per minute (e.g. computed as described below with reference toFIG. 7 o) by the typical respiratory rate in breaths per minute.

Following step 7440, the method 7400 computes three fuzzy truthvariables. Step 7445 computes DerivIncreaseIsLarge as a generallyincreasing function of the DerivativeRange returned by the three-statedetection step 7420. In one implementation, step 7445 computesDerivIncreaseIsLarge as follows:

DerivIncreaseIsLarge=FuzzyMember(DerivativeRange,0.25,0,0.5,1)  (21)

Step 7450 computes LateProportionIsLarge as a generally increasingfunction of LateProportion. In one implementation, step 7450 computesLateProportionIsLarge as follows:

LateProportionIsLarge=FuzzyMember(LateProportion,0.55,0,0.7,1)  (22)

Step 7455 computes Tidal VolumeIsNotLarge as a generally decreasingfunction of the inspiratory tidal volume computed at step 7430, withthresholds proportional to the typical recent tidal volume computed atstep 7440. In one implementation, step 7455 computesTidalVolumeIsNotLarge as follows

TidalVolumeIsNotLarge=FuzzyMember(InspTidalVolume,TypicalTidalVolume*1.3,1,TypicalTidalVolume*2.0,0)  (23)

Step 7460 computes a real-valued variable NoRecentJamming as a generallydecreasing function of the fuzzy truth variable RecentJamming computedby the jamming pre-processing algorithm 4319. In one implementation,step 7460 computes NoRecentJamming as follows:

NoRecentJamming=Interp(RecentJamming,0.25,1,0.5,0)  (24)

Finally, the method 7400 at step 7470 computes a variableReverseChairnessCurrent, the fuzzy extent to which the currentinspiration is reverse-chair-shaped, as the fuzzy And ofDerivIncreaseIsLarge, LateProportionIsLarge, and TidalVolumeIsNotLarge,multiplied by NoRecentJamming:

ReverseChairnessCurrent=FuzzyAnd(DerivIncreaseIsLarge,LateProportionIsLarge,TidalVolumeIsNotLarge)*NoRecentJamming  (25)

Thus the current inspiration is reverse-chair-shaped to the extent thatan increase in the derivative after the flat section is moderatelylarge, the proportion of the tidal volume in the latter half of thebreath is large, the tidal volume is not large (to help excludemicroarousals and sighs), and there has not been any recent jamming.

Detection of Apneas and Hypopneas 4325

In one form of the present technology, a processor 4230 executes one ormore algorithms 4325 for the detection of apneas and/or hypopneas.

Total upper airway obstruction produces zero true respiratory airflow.In the presence of ventilatory support, the respiratory airflow asestimated by the ventilator will in general not be zero, even in theabsence of leak. The rise in pressure during inspiration in the airpathand the mask results in compression of gas in the airpath, down to thesite of upper airway obstruction, and thus there is a true, if smallinflow, into the system. In addition, the rise in pressure duringinspiration may cause part of the mask to move away from the face, evenwhile maintaining a seal sufficient to prevent any leak, which resultsin a further inflow of gas into the airpath during inspiration.Corresponding outflows occur during expiration. In addition, the modelof leak as a function of mask pressure may be imperfect, particularly athigher leak levels, so that even during zero true airflow the estimatedflow may be alternately positive (during inspiration) and negativeduring expiration. For these reasons a criterion of zero or almost zerorespiratory airflow for detecting apneas will often not be met duringtrue closed (i.e. obstructive) apnea, and is thus inappropriate.

Hence, in one form of the present technology, the criterion fordetecting apnea is that the airflow is low relative to typical recentairflow.

FIG. 7 e is a flow chart illustrating a method 7500 that may be used toimplement apnea detection as part of the algorithm 4325 in one form ofthe present technology. In the first step 7510, a measure of the currentrespiratory airflow is computed. In one implementation, step 7510computes the RMS value of the respiratory airflow over a short recentinterval, in one implementation equal to the last two seconds.

At step 7520, a measure of the typical recent airflow is computed. Inone implementation, the measure of typical recent airflow is computeddirectly, by calculating the RMS value of respiratory airflow in awindow of length longer than the interval used in step 7510, in oneimplementation 60 seconds before the present. In an alternativeimplementation, step 7520 calculates the square root of the output of alowpass filter on the square of the value of respiratory airflow, wherethe lowpass filter has a typical time response of the order of 60seconds, such as a first-order lowpass filter with time constant 60seconds.

At step 7530, the ratio of the measure of the current airflow to themeasure of typical recent airflow is computed. Step 7540 then testswhether the computed ratio is continuously less than or equal to a lowthreshold (0.25 in one implementation) for some duration that is greaterthan or equal to a predetermined duration D (10 seconds in oneimplementation). If so (“Y”), a Boolean variable (flag), Apnea,indicating whether an apnea was detected, is set to true at step 7550.Otherwise (“N”), the flag is cleared at step 7560. Contiguous periods oftime during which Apnea is true are regarded as apnea episodes.According to step 7540, apnea periods must be at least D in duration.

Under conditions of severe but not total upper airway obstruction, aventilator may produce some true respiratory flow, particularly if it isa ventilator that rapidly increases pressure support in response tohypopnea, as in one form of the present technology. Alternatively, whenleak is not well modelled, there may appear to be a modest respiratoryairflow, large enough for the method 7500 to not detect an apnea. Thetrue respiratory airflow may be low enough so that if respiratoryairflow estimation (algorithm 4318) had been accurate, the method 7500would have detected an apnea. En either “false negative” situation ofapnea detection, the combination of moderate to large pressure supportand small absolute airflow, referred to as high ventilation impedance,may be taken as an indication of a hypopnea.

FIG. 7 f is a flow chart illustrating a method 7600 that may be used toimplement hypopnea detection as part of the algorithm 4325 in one formof the present technology. The method 7600 starts at step 7610, whichapplies a lowpass filter with a characteristic response time on theorder of one or two typical breaths to the absolute value of airflow. Inone implementation of step 7610, the lowpass filter is a second orderBessel lowpass filter, implemented digitally using the bilineartransform method, with a frequency response having its −3 dB point at3.2/60 seconds. The output of step 7610 is denotedAbsAirflowFilterOutput.

At the next step 7620, the target absolute airflow (denotedTgtAbsAirflow), is computed as twice the current target ventilation, acontinuously changing quantity that is based on the measure of typicalrecent ventilation, and is computed using the algorithm 4328 describedbelow. Step 7620 then computes a fuzzy truth variable AirflowIsSmall,indicating the extent to which the absolute airflow is small, as agenerally decreasing function of AbsAirflowFilterOutput, with thresholdsproportional to TgtAbsAirflow. In one implementation, step 7620 computesAirflowIsSmall as follows:

AirflowIsSmall=FuzzyMember(AbsAirflowFilterOutput,TgtAbsAirflow*0.15,1,TgtAbsAirflow*0.25,0)  (26)

Step 7630 then computes a fuzzy truth variable SwingIsLarge indicatingthe extent to which the pressure support (i.e., the swing) is large. Inone implementation, step 7630 computes SwingIsLarge as follows:

SwingIsLarge=FuzzyMember(Swing,6,0,8,1)  (27)

A fuzzy truth variable VentilationImpedanceIsHigh indicative of highventilation impedance is calculated at step 7640 as the fuzzy “And” ofAirflowIsSmall and SwingIsLarge:

VentilationImpedanceIsHigh=FuzzyAnd(AirflowIsSmall,SwingIsLarge)  (28)

In a manner corresponding broadly to the apnea detection method 7500,contiguous periods of time during which VentilationImpedanceIsHigh isgreater than zero, i.e. is not fuzzily false, are regarded as hypopneaepisodes.

Detection of Snore 4326

In one form of the present technology, a processor 4230 executes one ormore snore algorithms for the detection of snore.

Snore is generally indicative of upper airway obstruction. A relativelysimple technique to obtain a snore signal may include applying abandpass filter to a pressure signal measured at a suitable location,typically in the ventilator airpath, and deriving an indicator of themagnitude of the filter output, for example by full-wave rectificationand low-pass filtering. Some compensation for noise produced by theventilator is typically necessary.

Mask leak, to some extent vent flow, and various other factors, mayproduce sounds (designated “spurious snore”) which this or other methodsof evaluating the degree of snore treat as snore. This can lead to a“positive feedback” situation in which EPAP is increased in response tomask leak, further increasing the amount of mask leak, which is treatedas snore, resulting in a further increase in EPAP, and so on.

The broad goal of the algorithm 4326 is to compute a measure of trueinspiratory snore and to detect apparent snore during expiration. Theaim is to provide an EPAP increase generally increasing with the degreeof inspiratory snore, but if the apparent expiratory snore is too large,not to provide any increase in EPAP, because the expiratory snore isvery likely to represent mask leak or possibly another source ofspurious snore. This means that true expiratory snore is not treated,but true expiratory snore appears to be quite rare, especially in aventilator rather than a CPAP device, and the resulting gain inspecificity is well worthwhile. True snore is generally inspiratoryonly, maximal in mid to late inspiration, and generally decreasesmarkedly or disappears in the last part of inspiration.

FIG. 7 g is a flow chart illustrating a method 7700 of computing ameasure of inspiratory snore and detecting apparent expiratory snore,that may be used to implement the algorithm 4326 in one form of thepresent technology.

Since there is no generally accepted standard for measuring snore, inthe following the magnitude of snore is expressed in “snore units”. Inthese units, 0 represents no snore, 0.2 represents a very soft snore,1.0 represents a moderately loud snore, and 2.0 a louder snore. Theseunits are linear in amplitude.

The method 7700 starts at step 7710, which applies a snore filter to theinstantaneous mask pressure Pm. In one implementation, the snore filteris a bandpass filter with passband between 30 and 300 Hz, followed byfull wave rectification and lowpass filtering with a high frequencycutoff of between 0.5 and 2 Hz. The output of the snore filter is termed“raw snore”.

At the next step 7720, a snore threshold is computed. The snorethreshold depends not on the EPAP, but on the instantaneous maskpressure Pm, because the spurious snore signal generally varies almostinstantaneously with mask pressure, possibly with a small delay due tophysical properties, such as inertia, of the mask and face. The snorethreshold tsn follows a generally increasing course with increasing maskpressure Pm. In one implementation, step 7720 computes tsn (in snoreunits) as follows:

tsn=Interp(Pm,8,0.20,10,0.25,12,0.30,14,0.40,16,0.60,18,1.00)  (29)

Step 7730 follows, at which the method 7700 computes a weighting W(s) tobe applied to the amount of raw inspiratory snore s above the snorethreshold tsn. If the inspiratory flow is high, the noise produced bothin the patient's respiratory system and in the airpath may beconsiderable, producing spurious snore. In such a situation, the highflow indicates that there cannot be any significant degree of UAO. Rawsnore occurring at very high respiratory flows is therefore given a lowweighting.

Because, as described above, due to uncompensated leak, there isgenerally some uncertainty about the baseline of respiratory flow (i.e.a calculated respiratory flow of 0 does not correspond exactly to zerotrue respiratory flow), raw snore occurring at very low respiratoryflows is also given a low weighting.

The weighting function W(s) computed at step 7730 is therefore, in oneimplementation, given by

W(s)=Interp(Qr,0.05,0,0.1,1,0.5,1,0.8,0)  (30)

Step 7735 accumulates the weighted difference between the amount of rawsnore and the snore threshold tsn over the inspiratory portion of thecurrent breath, by multiplying the difference at each sample (e.g. at 50Hz) by the weighting function W(s). Step 7740 then, at the end of theinspiratory portion of the current breath, divides the accumulatedweighted difference by the accumulation of W(s) over the inspiratoryportion. The result is the mean weighted inspiratory snore in excess ofthreshold (MWISAT) for the current breath.

The method 7700 uses joint thresholds on intensity and duration of rawsnore during the expiratory portion for detecting significant expiratorysnore. The thresholds are “joint” in the sense that the threshold onduration generally decreases as the threshold on intensity increases.This means that if there has been loud expiratory snore for a shortperiod of time, or softer expiratory snore for a longer period of time,or yet softer expiratory snore for a yet longer period of time,significant expiratory snore is deemed to be present. In oneimplementation, the durations are measured in terms of time, but inother implementations the durations are normalised by dividing by theduration Te of expiration.

Step 7750 of the method 7700 therefore accumulates a distribution D(s)(analogous to an observed probability distribution function) of theintensity s of raw snore during the expiratory portion of the currentbreath. In one implementation, step 7750 maintains D(s) as a histogramof raw snore intensities s during expiration. At the conclusion of thecurrent breath, step 7760 converts the distribution D(s) into a reversecumulative distribution function (CDF) C(s) of raw snore intensity sduring the expiration. The reverse CDF C(s) is the proportion of theexpiratory duration Te spent at a snore intensity greater than or equalto s. The reverse CDF C(s) is then, at step 7770, compared with apredetermined “critical” snore function Cc(s) that expresses the jointthresholds on intensity and duration. The critical snore function Cc(s)decreases generally with increasing raw snore intensity s. In oneimplementation, the critical snore function Cc(s) is defined as follows:

Cc(s)=Interp(s,0.2,1.0,0.5,0.3,1,0.1)  (31)

If the actual snore reverse CDF C(s) is above the critical snorefunction Cc(s) at any value of raw snore intensity s above a minimumintensity st of raw snore (i.e. if there exists s>st such thatC(s)>Cc(s)) (“Y”), step 7780 sets a Boolean variable ExpiratorySnoreindicating that significant expiratory snore has been detected to True.Otherwise (“N”), step 7790 sets ExpiratorySnore to False. In oneimplementation, the minimum intensity st of raw snore is 0.2 snoreunits.

For example, based on the definition of the critical snore functionCc(s) in equation (31), if expiratory snore intensity has been greaterthan or equal to 1 snore unit for 0.1 seconds, or if expiratory snoreintensity has been greater than or equal to 0.5 snore units for 0.3seconds, apparent expiratory snore is detected.

Determination of EPAP 4327

In one form of the present technology, a number of different featuresindicative of upper airway obstruction, if present, cause a rise in theEPAP above a pre-set minimum value minEPAP, to a degree which is broadlyproportional to the severity of the upper airway obstruction. When nofeatures indicative of UAO are present, the EPAP decays progressivelytowards the pre-set minimum EPAP. This decay tends to minimise the EPAPdelivered. At any given time, the EPAP is a balance between the forcestending to make it rise and the tendency to decay. An approximateequilibrium may be reached in which occasional indicators of mild UAOcause upward movements in EPAP which are counterbalanced by the decaythat occurs when there are no indicators of UAO.

The EPAP response to the indications of flow limitation is progressive(i.e., more flow limitation results in a greater EPAP component comparedto the EPAP component due to less flow limitation), because withprogressively more severe flow limitation the need to respond rapidly totry to prevent an apnea or arousal increases, and also because there isless uncertainty about the presence of flow limitation. Control systemswith progressive responses to signals are also almost invariably morestable and generally better behaved than those with large changes inresponse to small changes in the level of signals.

When the algorithm 4327 prescribes an increase in EPAP, that increasemay not occur instantaneously. Such rises in EPAP may be controlled bythe processor 4230 and timed to occur only during what the PAP device4000 considers to be inspiration. It is believed that rises in EPAPduring expiration are more prone to cause arousals than the same risesduring inspiration, probably because a rise in inspiration decreasesinspiratory work, but a rise in expiration tends to push the patientinto the next inspiration. An example of such a technique is disclosedin U.S. Patent Application Publication No. 2011/0203588 A1, thedisclosure of which is incorporated herein by reference.

FIG. 7 h is a flow chart illustrating a method 7800 of determining a newvalue of EPAP, CurrentEPAP, as a function of the various indications ofupper airway obstruction computed by the algorithms 4324, 4325, and4326. The method 7800 may be used to implement the algorithm 4327 in oneform of the present technology.

The method 7800 computes five separate components of EPAP above thepre-set minimum value minEPAP: EPAP_((1,2)) (due to apnea and/or highventilation impedance) at step 7810, EPAP₍₃₎ (due to flatness ofinspiratory flow) at step 7820, EPAP₍₄₎ (due to M-shaped inspiratoryflow) at step 7830, EPAP₍₅₎ (due to reverse chairness of inspiratoryflow) at step 7840, and EPAP₍₆₎ (due to snore) at step 7850. Step 7860adds these five components to the pre-set minimum value minEPAP. Finallyat step 7870, the method 7800 ensures that the resulting new value ofCurrentEPAP does not exceed a pre-set maximum value maxEPAP. In otherwords, step 7870 “clips above” the newly computed value of CurrentEPAPto maxEPAP. The method 7800 then concludes.

Each of the steps 7810 to 7850 takes as input, in addition to thecorresponding measure(s) of UAO, one or more of the following PAP devicevariables or signals: the respiratory flow Qr, the amount Leak of leak(equal to the leak flow Ql, in litres per second), the current targetventilation Vtgt, the present value of CurrentEPAP, the amount of swing(or pressure support), the instantaneous mask pressure Pm, and therecent jamming fuzzy truth variable RecentJamming.

In general, it makes sense to require stronger evidence of UAO for thesame rise in EPAP as the current value of EPAP increases, because thepotential adverse consequences of raised EPAP increase as the EPAPincreases. These consequences are that the maximum possible pressuresupport, given a fixed maximum pressure, decreases, and leak becomesmore likely. As leak increases, the level of confidence in the accuracyof the calculated respiratory flow waveform decreases, because leakmodels tend to become increasingly inaccurate as the magnitude of theleak increases.

EPAP Component Due to Apnea/Hypopnea

In step 7810, the EPAP component EPAP_((1,2)) increases with theduration of the detected episode of apnea or high ventilation impedance(HVI). Since episodes of apnea and high ventilation impedance ascalculated by the algorithm 4325 may overlap, it is desirable to combinethem in some way.

FIG. 7 i is a flow chart illustrating a method 7900 which may be used toimplement step 7810 of the method 7800. The method 7900 starts at step7910, which determines whether an episode of apnea or HVI, i.e. a periodduring which it is continuously the case that FuzzyOr(VentilationImpedanceIsHigh, Apnea)>0 (here taking Apnea to be a fuzzytruth variable which is either 0 or 1), has just ceased. If so (“Y”),the next step 7920 computes the duration T_apn_Rx of the episode fortherapy purposes. In one implementation of step 7920, the duration iscomputed by calculating a weight function W(t) for each time t asfollows: where Apnea is true, W(t)=1; when Apnea is false butVentilationImpedanceIsHigh is not fuzzily false, W(t) depends on thevalue of VentilationImpedanceIsHigh, for example W(t) is a scalingfactor multiplied by VentilationImpedanceIsHigh. The integral withrespect to time of W(t) over the episode may then be taken as theduration T_apn_Rx of the combined apnea and high ventilation impedanceepisode for therapy purposes.

Another implementation of step 7920, which is simpler and moreconservative, is as follows. If there was an apnea during the episode,the period of high ventilation impedance is ignored and T_apn_Rx istaken just to be the actual apnea duration as described above.Otherwise, T_apn_Rx is set equal to the weighted duration of highventilation impedance, determined by integratingVentilationImpedanceIsHigh as described above, multiplied by a scalingfactor.

In either implementation of step 7920, the scaling factor is set to bebetween 0 and 1, for example 0.75, due to the fact that when only thestate of high ventilation impedance exists, either the hypopnea is notas severe as that which obtains when the apnea detection method 7500detects an apnea, or that there has actually been an apnea, but there islower confidence that this is the case, or some combination of these twopossibilities, so the hypopnea deserves less therapy than a clearlydiagnosed apnea of the same duration.

In steps 7930 and 7940, the EPAP component EPAP_((1,2)) due toapnea/hypopnea is computed in such a way that with increasing T_apn_Rx,the maximum possible new value of EPAP as a result of EPAP_((1,2)),termed MaxPossibleNewEPAP, exponentially approaches a value, termedHighApneaRollOffPressure, that is set somewhat above the maximumpossible EPAP value maxEPAP. In one implementation of step 7930,

HighApneaRollOffPressure=maxEPAP+2  (32)

and

MaxPossibleNewEPAP=CurrentEPAP+(HighApneaRollOffPressure−CurrentEPAP)*(1−exp(k*T_apn_(—)Rx))  (33)

The rate constant k in equation (33) (with units 1/sec) is decreased asHighApneaRollOffPressure increases, to avoid too rapid an increase inpressure at low EPAP values. In one implementation of step 7930,

k=1/45*10/HighApneaRollOffPressure  (34)

The actual new EPAP as a result of EPAP_((1,2)) is then limited at step7940 to be no more than maxEPAP. The component EPAP_((1,2)) is thereforecomputed at step 7940 as

EPAP_((1,2))=min(MaxPossibleNewEPAP,maxEPAP)−CurrentEPAP  (35)

If step 7910 returns “N”, i.e. no increase in EPAP due to apnea/hypopneais prescribed, at step 7950 the EPAP component EPAP_((1,2)) is decayedexponentially towards zero using a time constant τ_(1,2). This isaccomplished by reducing EPAP_((1,2)) by EPAP_((1,2))*ΔT/τ_(1,2), whereΔT is the interval since the last update of EPAP_((1,2)). In oneimplementation, the time constant τ_(1,2) is 40 minutes.

EPAP Component Due to Flatness

FIG. 7 j is a flow chart illustrating a method 71000 that may be used toimplement step 7820 of the method 7800.

As the value of CurrentEPAP rises, the level of flatness of inspiratoryflow limitation required for any increase in the EPAP component EPAP₍₃₎due to flatness increases. The method 71000 therefore starts at step71010, which calculates a value CurrentEEP_RxFactor that generallydecreases as CurrentEPAP increases. In one implementation,

CurrentEEP_RxFactor=Interp(CurrentEPAP,12,1,16,0.6)  (36)

In addition, the increase in EPAP₍₃₎ is decreased progressively as theamount of leak increases. The method 71000 therefore at step 71020computes a variable LeakRxFactor which generally decreases as Leakincreases. In one implementation, step 71020 computes LeakRxFactor asfollows:

LeakRxFactor=Interp(Leak,0.5,1,1,0)  (37)

The thresholds 0.5 and 1.0 on Leak in equation (37) are higher than inprevious technology.

In addition, as “valve-like leak”, an indicator of mouth leak,increases, the minimum level of flatness required for any increase inEPAP₍₃₎ increases. Step 71030 therefore computes a variableEarlyExpLeakRatio as the ratio of the peak flow in the first 0.5 secondsof expiration to the mean flow in the next 0.5 seconds of expiration.During valve-like leaks, EarlyExpLeakRatio typically exceeds 5:1. Normalbreathing gives a ratio of about 1:1 to 4:1. Step 71030 then calculatesa variable ValveLikeLeak_RxFactor that generally decreases asEarlyExpLeakRatio increases above the thresholds that indicatevalve-like leak is likely to be happening. In one implementation,

ValveLikeLeak_RxFactor=Interp(EarlyExpLeakRatio,4,1,5,0)  (38)

From these three factors, step 71040 calculates a thresholdMinFlatnessForRx on flatness for any increase in EPAP₍₃₎ to beprescribed, as follows:

MinFlatnessForRx=1−LeakRxFactor*ValveLikeLeak_RxFactor*CurrentEEP_RxFactor  (39).

According to equations (36) to (39), if any of CurrentEPAP, Leak, orvalve-like leak is large, the threshold MinFlatnessForRx is close to 1,and hence only severe flatness will cause any increase in EPAP₍₃₎.

Step 71050 then tests whether the value of Flatness computed by thealgorithm 4324 is less than or equal to the threshold MinFlatnessForRx.If not (“N”), the increase dEPAP₍₃₎ in EPAP₍₃₎ is calculated at step71060 in proportion to the excess of Flatness over the thresholdMinFlatnessForRx. In one implementation, the constant of proportionalityis 0.5 cm H₂O:

ΔEPAP₍₃₎=(Flatness−MinFlatnessForRx)*0.5.  (40)

Step 71060 then increases EPAP₍₃₎ by ΔEPAP₍₃₎. Step 71070 clips theincreased value of EPAP₍₃₎ to maxEPAP−CurrentEPAP, to ensure theincreased value of EPAP as a result of flatness does not exceed maxEPAP.

If step 71050 determines that Flatness is less than or equal toMinFlatnessForRx (“Y”), at step 71080 the value of EPAP₍₃₎ is decayedexponentially towards zero using a time constant τ₃. This isaccomplished by reducing EPAP₍₃₎ by EPAP₍₃₎*ΔT/τ₃, where ΔT is theinterval since the last update of EPAP₍₃₎. In one implementation, thetime constant τ₃ is 20 minutes.

EPAP Component Due to M-Shape

FIG. 7 k is a flow chart illustrating a method 71100 that may be used toimplement step 7830 of the method 7800 in one form of the presenttechnology.

Very long inspiratory flow waveforms may have an approximately M-shapedappearance, but this rarely reflects the flow-limited breathing. Hence,the method 71100 starts at step 71110, which tests whether the durationTi of inspiration is greater than a “long” threshold, 3.5 seconds in oneimplementation. If so (“Y”), a variable MRxProportion, the proportion ofthe maximum increase per breath in EPAP₍₄₎, the EPAP component due toM-shaped inspiratory flow, to be applied in the current breath, is setto 0 at step 71120. Otherwise (“N”), step 71130 computes MRxProportionto increase generally with the value of M3RatioSym computed by thealgorithm 4324. In one implementation, step 71130 computes MRxProportionfrom M3RatioSym as follows:

MRxProportion=Interp(M3RatioSym,0.17,0,0.3,1)  (41)

Breaths with ventilations significantly larger than the typical recentventilation rarely reflect flow limitation, and are usually behavioural.Therefore, following either step 71120 or step 71130, step 71040calculates the ratio of the breathwise ventilation (the mean of theinstantaneous ventilation Vent over the breath) to the typical recentventilation (e.g. computed as described below with reference to FIG. 7o). Step 71140 then adjusts MRxProportion to generally decrease as thatratio increases. In one implementation, step 71140 adjusts MRXProportionas follows:

MRxProportion:=MRxProportion*Interp(BreathwiseVentilation/TypicalRecentVentilation,1.1,1,1.3,0)  (42)

Step 71150 tests whether M3RatioSym is greater than 0. If so (“Y”), step71160 increases EPAP₍₄₎ by an amount dEPAP₍₄₎ proportional toMRxProportion. The constant of proportionality, i.e. maximum increaseper breath in the EPAP component due to M-shaped inspiratory flow, inone implementation, is set to 0.3 cm H₂O:

ΔEPAP₍₄₎=MRxProportion*0.3.  (43)

Step 71170 clips the increased value of EPAP₍₄₎ to maxEPAP−CurrentEPAP,to ensure the new value of EPAP does not exceed maxEPAP.

If step 71150 determines that M3RatioSym is not greater than zero (“N”),at step 71180 the value of EPAP₍₄₎ is decayed exponentially towards zerousing a time constant τ₄. This is accomplished by reducing EPAP₍₄₎ byEPAP₍₄₎*ΔT/τ₄, where ΔT is the interval since the last update ofEPAP₍₄₎. In one implementation, the time constant τ₄ is 20 minutes.

EPAP Component Due to Reverse Chairness

FIG. 7 l is a flow chart illustrating a method 71200 that may be used toimplement step 7840 of the method 7800.

As the current value of EPAP increases, it is desirable to have greaterconfidence that the underlying state which causes reverse chairness isindeed present, before further increasing the EPAP. One way to achievethis is to assess the extent to which the preceding breath also exhibitsreverse chairness, and then, as the current EPAP rises, increasinglyfavour that measure of “consistency” over the simple reverse chairnessof the current breath when computing the increase in EPAP.

The first step 71210 of the method 71200 therefore calculates a variableReverseChairnessConsistent as a weighted geometric mean ofReverseChairnessCurrent computed by the algorithm 4324 for the currentand preceding breaths. This calculation can be interpreted as aparticular kind of fuzzy “and” function over current and precedingbreaths.

In one implementation, step 71210 finds the minimum and maximum of thevalues of ReverseChairnessCurrent for the current and preceding breaths,designating them MinChairness and MaxChairness. If either or both ofMinChairness and MaxChairness is zero, the reverse chairness measureReverseChairnessConsistent is set to zero. Otherwise, in oneimplementation step 71210 calculates ReverseChairnessConsistent asfollows:

ReverseChairnessConsistent=exp(0.6*log(MinChairness)+0.4*log(MaxChairness))  (44)

Step 71220 then computes a variable ReverseChairnessForRx, a measure ofreverse chairness for therapy purposes, that transitions fromReverseChairnessCurrent to ReverseChairnessConsistent as CurrentEPAPincreases. In one implementation, step 71220 computesReverseChairnessForRx as

ReverseChairnessForRx=Interp(CurrentEPAP,8,ReverseChairnessCurrent,10,ReverseChairnessConsistent)  (45)

Step 71230 then tests whether ReverseChairnessForRx is less than a lowthreshold, 0.05 in one implementation. If not (“N”), the reversechairness is deemed significant, step 71240 increases the EPAP componentEPAP₍₅₎ due to reverse chairness by an amount ΔEPAP₍₅₎ that isproportional to ReverseChairnessForRx by an amount that decreases withincreasing current EPAP and increasing leak. In one implementation, step71240 increases EPAP₍₅₎ by

ΔEPAP₍₅₎=0.2*Interp(CurrentEPAP,10,1,20,0)*Interp(Leak,0.5,1,1,0)*ReverseChairnessForRx  (46)

Step 71250 clips the increased value of EPAP₍₅₎ to maxEPAP−CurrentEPAP,to ensure the new value of EPAP does not exceed maxEPAP.

If step 71230 determines that ReverseChairnessForRx is insignificant(“N”), at step 71260 the value of EPAP₍₅₎ is decayed exponentiallytowards zero using a time constant τ₅. This is accomplished by reducingEPAP_((S)) by EPAP₍₅₎*ΔT/τ₅, where ΔT is the interval since the lastupdate of EPAP₍₅₎. In one implementation, the time constant τ₅ is 20minutes.

EPAP Component Due to Snore

FIG. 7 m is a flow chart illustrating a method 71300 that may be used toimplement step 7850 of the method 7800.

The method 71300 start at step 71320, which examines the Booleanvariable ExpiratorySnore, indicating that significant expiratory snorehas been detected, computed by the algorithm 4326. If step 71320determines that ExpiratorySnore is true (“Y”), at step 71330 the valueof the component EPAP₍₆₎ of EPAP due to snore is decayed exponentiallytowards zero using a time constant τ₆. This is accomplished by reducingEPAP₍₆₎ by EPAP₍₆₎*ΔT/τ₆, where ΔT is the interval since the last updateof EPAP₍₆₎. In one implementation, the time constant τ₆ is 20 minutes.

Otherwise (“N”), step 71340 determines whether the mean weightedinspiratory snore above the threshold (MWISAT) computed by the algorithm4326 is greater than zero, indicating inspiratory snore is present. Ifnot (“N”), the method 71300 proceeds to step 71330 to decay the value ofEPAP₍₆₎ towards zero as described above.

Otherwise (“Y”), the EPAP component EPAP₍₆₎ is increased according tothe MWISAT value. As explained earlier, when there is jamming, there isgreater uncertainty about the respiratory flow baseline. Hence theamount of increase in EPAP₍₆₎ is decreased with increasing jamming, andin particular with the maximum value of the fuzzy truth variableRecentJamming during the breath just completed. This maximum value,MaxJammingDuringBreath, is computed from RecentJamming at step 71350.

Step 71360 then increases EPAP₍₆₎ by an amount ΔEPAP₍₆₎ that isproportional to MWISAT by an amount that decreases asMaxJammingDuringBreath increases. In one implementation, step 71360increases EPAP₍₆₎ by

ΔEPAP₍₆₎=1.5 cmH₂O*Interp(MaxJammingDuringBreath,0.15,1,0.3,0)*MWISAT  (47)

Finally, step 71370 clips the increased value of EPAP₍₆₎ tomaxEPAP−CurrentEPAP, to ensure the new value of EPAP does not exceedmaxEPAP.

Determination of Target Ventilation 4328

In previous approaches, the target ventilation has been set to 90% ofthe typical recent ventilation, calculated as the output of afirst-order lowpass filter with time constant 3 minutes (the ventilationfilter) that is applied to the instantaneous ventilation.

Under such approaches, there is a fundamental asymmetry in the dynamicsof target ventilation with respect to increases and decreases. Theventilator does nothing to counteract a rise in ventilation (it merelylowers its pressure support to minimum), but counteracts a fall inventilation by supporting the ventilation at 90% of the typical recentvalue. Consider the simple case of a sudden reduction in patient effortto zero. If the maximum pressure support were zero, the actualventilation would be zero, and so the ventilation filter output wouldfall towards zero with a time constant of 3 minutes. However, if maximumpressure support is adequate to maintain the target ventilation, theactual ventilation is maintained at 90% of the value just before patienteffort dropped to zero. Hence the difference between the output and theinput of the ventilation filter is 10% of what it would have been hadthere been no pressure support, and so the rate of fall of the outputvalue is 10% of what it would have been, causing the ventilation todecline with a time constant of 1/0.1*3=30 minutes.

Because it is easy for target ventilation to rise, but hard for it tofall, brief rises in actual ventilation produce long-lived rises intarget and actual ventilation. Such rises are typically due either toarousals during sleep or to awake breathing, where a brief rise inventilation may be associated with the effort of getting into bed ormoving around in bed, or to anxiety produced by the ventilatoraggressively supporting ventilation during the brief apneas andhypopneas which are normal at sleep onset, but which the ventilatortreats as the potential onset of a cycle of periodic breathing. Inaddition, when the ventilator's target ventilation is above the actualventilation, the ventilator becomes increasingly insistent on deliveringbreaths at the standard rate, which in an awake patient, can causefurther discomfort, anxiety, and fighting the ventilator followed bybursts of hyperventilation, which further raises the target ventilationand thus exacerbates the situation. Moreover, an aim of the presenttechnology is to stabilise the ventilation, not to set any particularlevel, and the patients in whom it is generally used have arterial CO₂levels below normal, with the goal in these patients being to raise theCO₂ level, so it is desirable to maintain pressure support at the lowestlevel consistent with awake comfort.

To this end, the present technology contains features designed to makeit harder for the target ventilation to rise rapidly, and to make iteasier for the target ventilation to fall when pressure support has beenreasonably stable for a while, and hence by the above considerations isat an inappropriately high level.

FIG. 7 n is a flow chart illustrating a method 71400 of computing thetarget ventilation, that may be used to implement the algorithm 4328 inone form of the present technology.

The method 71400 starts at step 71410, which computes a measure of thetypical recent ventilation from the instantaneous ventilation (computedby the algorithm 4323), as described in detail below with reference toFIG. 7 o. Step 71410 is sometimes referred to as the typical recentventilation filter. The following step 71420 computes a fuzzy truthvariable ShouldSpeedUpTargetVentilationAdjustment, the fuzzy extent towhich any fall in target ventilation should be speeded up, from thecurrent value of pressure support, as described below with reference toFIG. 7 p. Step 71430 then computes a target fraction that is to bemultiplied by the typical recent ventilation. In previous approaches,the target fraction was fixed at a value just below 1, e.g. 0.9. Onemechanism for lowering the target ventilation more rapidly involvesdecreasing the target fraction to a value slightly further below 1 asShouldSpeedUpTargetVentilationAdjustment increases. In oneimplementation, step 71430 computes the target fraction as

Interp(ShouldSpeedUpTargetVentilationAdjustment,0,0.9,1,0.8)  (48)

which has the effect of producing a target fraction of 0.8 whenShouldSpeedUpTargetVentilationAdjustment is fully true.

The next step 71440 multiplies the computed target fraction by themeasure of typical recent ventilation computed by step 71410. Theresulting product is passed to step 71460 that computes the targetventilation, as described in detail below with reference to FIG. 7 q.Step 71460 is sometimes referred to as the target ventilation filter.The rate constant (the reciprocal of the time constant) of the low passfilter that computed the target ventilation in previous approaches wasfixed, typically at 1/180, and equal for both increases and decreases intarget ventilation. However, another mechanism for lowering the targetventilation more rapidly is to increase the decreasing rate constant ofthe target ventilation filter asShouldSpeedUpTargetVentilationAdjustment increases. Step 71450 thereforecomputes a factor SpeedUpRatio that generally increases withShouldSpeedUpTargetVentilationAdjustment, to be multiplied by thedecreasing rate constant in step 71460. In one implementation, step71450 computes the factor SpeedUpRatio as follows:

SpeedUpRatio=1+2*ShouldSpeedUpTargetVentilationAdjustment  (49)

so that when ShouldSpeedUpTargetVentilationAdjustment is fully true, thedecreasing rate constant has a maximum value of 3 times its basic value.

The combination of these two mechanisms (equations (48) and (49)) canthus produce a speedup in downward adjustment of target ventilation by afactor of 6.

The time taken to reduce the target ventilation to the patient's meanventilation requirement depends on how much the target ventilation isabove this requirement, but it is not unusual to see a reduction intarget ventilation over a period of 0.1 to 3 minutes (after the initial90 seconds of stable nontrivial pressure support) such that the targetand hence actual ventilation is lowered to a level which results in thearterial CO₂ being above the apneic threshold, so that intrinsicrespiratory drive returns, and thus pressure support drops rapidly tominimum.

FIG. 7 o is a flow chart illustrating a method 71500 of computing ameasure of the typical recent ventilation, as used to implement step71410 in the method 71400 in one form of the present technology.

Jamming (described above), by shifting the respiratory flow baseline,almost always causes an unjustified increase in apparent ventilation,and therefore the typical recent ventilation. Thus, in one form of thepresent technology, the rate of adjustment of typical recent ventilationis reduced when there is, or has recently been, jamming.

The instantaneous ventilation (computed by the algorithm 4323) is inputto a jam-dependent lowpass filter 71510 that comprises the steps 71520to 71580, executed on receipt of each input sample. The jam-dependentfilter 71510 effectively slows down time to the extent that there is, orhas recently been, jamming. The time-slowing in the jam-dependent filter71510 is implemented by accumulating the proportion of an update whichshould be performed to the jam-dependent filter output, and allowing theupdate to occur only when that accumulated proportion exceeds one. Therate of updating of output samples of the jam-dependent filter isthereby reduced by the value of the update proportion. The first step71520 therefore computes a variable UpdateProportion that generallydecreases from 1 to 0 as RecentJamming increases. In one implementation,step 71520 computes UpdateProportion as

UpdateProportion=Interp(RecentJamming,0.1,1,0.3,0)  (50)

The next step 71530 increments an accumulated value of UpdateProportionby UpdateProportion. Step 71540 then tests whether the accumulated valueof UpdateProportion is greater than or equal to one. If not (“N”), avariable WeightedSum is incremented by the product of UpdateProportionand the ventilation filter's current output sample (step 71550). Themethod 71500 then returns to step 71520 to compute a new value ofUpdateProportion from RecentJamming.

When the accumulated value of UpdateProportion is at least equal to 1(“Y”), a variable WeightedVentilation is computed at step 71560 as thesum of WeightedSum and the product of the ventilation filter's currentoutput sample and one minus the previous value of the accumulatedUpdateProportion (which was less than one).

Next, at step 71570 the accumulated value of UpdateProportion isre-initialized (to a value between 0 and 1) by subtracting one from theaccumulated value of UpdateProportion. Finally, step 71580re-initializes WeightedSum by multiplying the new value of theaccumulated UpdateProportion by the ventilation filter's current outputsample. The method 71500 then returns to step 71520 to compute a newvalue of UpdateProportion from RecentJamming.

The output of the jam-dependent filter 71510 is the sequence of valuesof WeightedVentilation produced by step 71560. If UpdateProportion is 1(as when RecentJamming is fully false), the output of the jam-dependentfilter 71510 is simply the instantaneous ventilation. IfUpdateProportion becomes zero (as when RecentJamming is fully true), theoutput of the jam-dependent filter 71510 is frozen at its current value.In the intermediate case where UpdateProportion is between 0 and 1, (aswhen RecentJamming is between 0.1 and 0.3), say 1/N where N is aninteger, the output of the jam-dependent filter 71510 is an N-sampleaverage of the instantaneous ventilation, updated once every N samples.

Because there is necessarily some delay in deciding whether jamming ispresent (for example, at quiet end-expiration, a sudden rise in leak isindistinguishable from true inspiratory flow; the difference takes alittle while to become apparent), the output of the jam-dependent filter71510 is, in one form of the present technology, passed to a ventilationfilter 71590 whose response has a similar time course to the jammingdetection algorithm 4319. Hence the output of the ventilation filter71590 does not rise in response to a sudden uncompensated leak untilRecentJamming starts to become fuzzily true. The output of theventilation filter 71590 is then the typical recent ventilation.

In one implementation, the ventilation filter 71590 is a second-orderBessel lowpass filter with a minus 3 db point of 0.0178 Hz. In otherimplementations, the response of the ventilation filter 71590 is slowenough to reduce within-breath fluctuations in ventilation to a valuemuch lower than the upward slew rate limit described below, and fastenough that its time constant is less than the three-minute timeconstant used in the typical recent ventilation filter of previousapproaches.

In an alternative implementation of step 71410, the ventilation filter71590 precedes the jam-dependent filter 71510. The output of thejam-dependent filter 71510 is then used as the measure of typical recentventilation.

Despite imposing a maximum limit on the rate of increase of targetventilation as described below, it is still relatively easy for thetarget ventilation to be above what the patient actually requires inmean. This may occur when target ventilation rises due to arousals, ormay simply be a result of the wake to sleep transition, when bothmetabolic rate decreases and respiratory controller CO₂ responsedecreases, and “awake drive” disappears. Again, an aim of the presenttechnology is to deliver pressure support above minimum only whenactually necessary to deal with relatively brisk and brief falls incentral drive, in order to stabilise ventilation in a respiratory systemin which ventilation would otherwise oscillate.

Hence, as described above, the method 71400 incorporates a step 71420 ofdetecting a state of fairly stable pressure support significantly abovethe minimum, and mechanisms (equations (48) and (49)) to speed targetventilation adjustment downwards when that occurs.

FIG. 7 p is a flow chart illustrating a method 71600 of computing thefuzzy truth variable ShouldSpeedUpTargetVentilationAdjustment as used atstep 71420 of the method 71400 in one form of the present technology.

Broadly, the method 71600 computes the fuzzy extent to which thepressure support above minimum (the “servoassistance” or “servo swing”)has been fairly stable for a first recent period and also for a secondrecent period substantially shorter than the first recent period, thenpressure support has been fairly stable for a while and is currentlyfairly stable. This could be determined by a variety of statisticalmeasures of spread, such as standard deviation, mean absolute deviation,or a high pass filter of some type; a fairly low value of spreadobtained by any of these indicates that the pressure support is fairlystable. In the method 71600, order statistics are used to determinestability, generally being more robust, especially when the distributionin the particular individual is unknown, as it typically is in thiscase.

In one implementation of the method 71600, the first recent period isthe most recent 90 seconds and the second recent period is the mostrecent 30 seconds. The choice of 90 seconds as the period over which toassess stability is determined by the fact that essentially allCheyne-Stokes oscillations of central drive have a period of 90 secondsor less, 40 to 60 seconds being the usual range. Periodic breathing ofother causes tends to have periods of 60 seconds or less. Thus if theventilator were delivering significant servoassistance only to stabilisesuch oscillations, it could not be fairly stable over a period of 90seconds.

The method 71600 starts at step 71610, at which the pressure supportabove minimum is lightly lowpass filtered, in one implementation with atime constant of 2 seconds. The next step 71620 calculates running orderstatistics over the most recent 30 seconds. In one implementation ofstep 71620, a histogram of values over the most recent 30 seconds iscontinually updated by means of a circular buffer of input values 30seconds in length. When a new input value is to be added, the histogramcategories of the newest and oldest sample in the circular buffer aredetermined, the count in the histogram category of the oldest sample isdecremented by one and the count in the histogram of the newest sampleis incremented by one. Determination of approximate order statisticsfrom histograms is routine. In particular, step 71620 compute's ameasure of spread referred to as Spread30 as the difference between the0.8 and the 0.2 order statistic, equivalently the difference between the80th percentile value and the 20th percentile value. Step 71620 alsocomputes the median, referred to as Median30. The following step 71630computes the ratio of Spread30 to Median30.

Step 71640 follows, at which running order statistics over the mostrecent 90 seconds are calculated in similar fashion to step 71620. Inparticular, step 71640 computes a measure of spread referred to asSpread90 as the difference between the 0.8 and the 0.2 order statistic,equivalently the difference between the 80th percentile value and the20th percentile value.

The final step 71650 computes the fuzzy truth variableShouldSpeedUpTargetVentilationAdjustment based on the computed orderstatistics and the constant MaxPossibleServoAssistance, the differencebetween maximum and minimum pressure support.

If Median30 is less than a low threshold value, set to 2 in oneimplementation, step 71650 sets ShouldSpeedUpTargetVentilationAdjustmentto zero, because recent pressure support cannot be stable and high underthese circumstances. Otherwise, step 71650 computesShouldSpeedUpTargetVentilationAdjustment as the fuzzy “And” of fivefuzzy truth variables. The first of the five fuzzy truth variables thatis fuzzy “Anded” to compute ShouldSpeedUpTargetVentilationAdjustmentindicates the extent to which MaxPossibleServoAssistance is largecompared to predetermined thresholds. This variable is present becausewhen MaxPossibleServoAssistance is small, it is fairly easy for theservoassistance to be small even in the presence of large fluctuationsin actual ventilation, so any apparent stability in pressure support isdiscounted.

The next two fuzzy truth variables evaluate the extent that pressuresupport has been fairly stable over the most recent 30 seconds. Thefourth fuzzy truth variable evaluates the extent to whichservoassistance has been nontrivial (e.g., generally sufficient toaffect the patient's respiratory pattern) over the last 30 seconds, andthe last fuzzy truth variable evaluates the extent to which pressuresupport has been stable over the last 90 seconds.

In one implementation step 71650 computesShouldSpeedUpTargetVentilationAdjustment as follows:

ShouldSpeedUpTargetVentilationAdjustment=FuzzyAnd(FuzzyMember(MaxPossibleServoAssistance,5,0,7,1),FuzzyMember(Spread30/Median30,0.25,1,0.5,0),FuzzyMember(Spread30,2,1,5,0),FuzzyMember(Median30,2,0,6,1),FuzzyMember(Spread90,2,1,5,0))  (51)

Order statistics are cleared at each “mask-on” event, so that from 90seconds after such an event, step 71650 is permitted to compute thevariable ShouldSpeedUpTargetVentilationAdjustment.

FIG. 7 q is a flow chart illustrating a method 71700 of computing thetarget ventilation from the typical recent ventilation, which may beused to implement step 71460 of the method 71400 in one form of thepresent technology.

The method 71700 imposes an upper limit on the rate of increase (theupward slew rate) of the target ventilation.

The method 71700 starts at step 71710, which subtracts the current valueof target ventilation from the typical recent ventilation multiplied bythe target fraction, as provided by step 71440 of the method 71400,yielding a prospective increment to the target ventilation.

Step 71720 determines whether the prospective increment is greater thanzero. If so (“Y”), the prospective increment is multiplied at step 71730by the increasing rate constant, which in one implementation is set to afixed value, typically 1/180 sec⁻¹. The next step 71740 clips theresulting adjusted increment above to the upward slew rate limit, whichin one implementation is set at 0.93 litres/minute/minute, correspondingto a target ventilation increase of 2.5 litres/minute per three minutes.The method 71700 then proceeds to step 71790, described below.

If step 71720 determined that the prospective increment is not greaterthan zero (“N”), step 71750 multiplies the prospective increment(actually a decrement) by the decreasing rate constant, and thefollowing step 71760 multiplies the product by the SpeedUpRatio computedat step 71450 of the method 71400 to obtain the adjusted increment.

After either of steps 71740 and 71760, the method 71700 adds theadjusted increment to the current target ventilation to generate the newvalue of target ventilation. Step 71795 is an optional step describedbelow.

Determination of Therapy Parameters 4329

The processor 4230 executes one or more algorithms 4329 for thedetermination of therapy parameters.

In one form of the present technology, the algorithm 4329 receives as aninput one of more of the following:

-   -   i. A waveform value Π(Φ) in the range [0, 1] (from the algorithm        4322);    -   ii. A measure of instantaneous ventilation Vent (from the        algorithm 4323);    -   iii. A target ventilation Vtgt (from the algorithm 4328); and    -   iv. An EPAP value (from the algorithm 4327).

The algorithm 4329 first computes a pressure support value A that issufficient to increase the instantaneous ventilation to the targetventilation. In one implementation, the algorithm 4329 computes A inproportion to the integral of the difference between the targetventilation and the instantaneous ventilation:

$\begin{matrix}{A = {G{\int_{t}{( {{Vtgt} - {Vent}} ){t}}}}} & (52)\end{matrix}$

where G is the controller gain, typically set to 0.3 cm H₂Olitres/min/second. Note that the computed pressure support A is clippedto the range [minSwing, maxSwing].

In implementations of the algorithm 4329, other forms of controller areused to compute the pressure support value A, from the targetventilation and the instantaneous ventilation, for example,proportional, proportional-integral, proportional-integral-differential.

The algorithm 4329 then computes the target treatment pressure Pt usingthe following equation:

Pt=EPAP+A*Π(Φ)  (53)

In other forms of the present technology, the algorithm 4329 computesthe pressure support value A as in equation (52). The therapy enginemodule 4320 then outputs the EPAP, the waveform value, and the computedvalue of pressure support A. The control module 4330 then performs theremaining computation of the target treatment pressure Pt as describedabove.

In other forms of the present technology, the algorithm 4329 merelyoutputs the EPAP, the waveform value, the target ventilation, and theinstantaneous ventilation. The control module 4330 then performs theremaining computation of the target treatment pressure Pt as describedabove.

Patients with periodic breathing of central origin (such as CSR) rarelyhave significant respiratory insufficiency, because insufficiencydecreases plant gain and so tends to stabilise the patient, even in thepresence of high respiratory controller gain. However, this combinationdoes occur occasionally. The mechanism may involve a lung disease inwhich arterial oxygen saturations are relatively low withoutcorresponding increases in work of breathing, the steep part of theoxyhaemoglobin saturation curve increasing the plant gain and, withthese patients, operating in a region where oxygen is an important partof respiratory controller drive. Such patients may have centralbreathing instability in slow wave sleep, possibly exacerbated by somedegree of cardiac failure, combined with marked REM desaturation. Whileoxygen is the principal therapy for these patients, it may beinsufficient to stabilise the breathing stability, and the REMdesaturation may be ameliorated to some extent by ventilatory support.

The conventional approach to such patients is to set a high enough levelof minimum pressure support for there to be adequate ventilatory supportin REM to address the respiratory insufficiency, as there is no periodicbreathing during REM. However, this is unsatisfactory, because itdiminishes the range of pressure support available to counteract theventilatory instability at other times. A preferable approach is to seta minimum target ventilation. This may be implemented straightforwardlyin the method 71700, by inserting an optional step 71795 (shown as adashed box in FIG. 7 q) that bounds the target ventilation below by theset minimum target ventilation. In one implementation, the minimumtarget ventilation rises gradually from zero to its set level, to allowthe patient to get to sleep before the target ventilation is boundedbelow by the minimum target ventilation.

Another approach is to set a minimum target gross alveolar ventilation(as described in the commonly owned U.S. Patent Application PublicationNo. 20070163590 A1, the disclosure of which is incorporated herein byreference), and to combine the control methodology based on grossalveolar ventilation described in that disclosure with the controlmethodology of algorithm 4329 described above.

To combine the two methodologies, one implementation is to run both inparallel, and adjust the pressure support to some combination of thevalues of pressure support set by each methodology. In oneimplementation, the combination is the greater of the two values.

The advantage of this combined approach is that a small tidal volume ata high respiratory rate, which may produce a low gross alveolarventilation but an adequate total ventilation, gets an appropriateincrease in pressure support rather than being regarded as satisfactoryventilation and not producing any increase in pressure support. This isnot normally an issue in patients with typical central breathinginstability, such as Cheyne Stokes, whose respiratory rates aregenerally in the normal range, but is important in respiratoryinsufficiency, where rapid shallow breathing may occur.

Control Module 4330

A control module 4330 in accordance with one form of the presenttechnology receives as an input a target treatment pressure Pt, andcontrols a therapy device 4245 to deliver that pressure.

A control module 4330 in accordance with another form of the presenttechnology receives as inputs an EPAP, a waveform value, and a level ofpressure support, computes a target treatment pressure Pt as in equation(53), and controls a therapy device 4245 to deliver that pressure.

A control module 4330 in accordance with another form of the presenttechnology receives as an input an EPAP, a waveform value, a targetventilation, and an instantaneous ventilation, computes a level ofpressure support from the target ventilation and the instantaneousventilation as in equation (52), computes a target treatment pressure Ptusing the EPAP, the waveform value, and the pressure support as inequation (53), and controls a therapy device 4245 to deliver thatpressure.

Detection of Fault Conditions 4340

In one form of the present technology, a processor executes one or moremethods for the detection of fault conditions. Preferably the faultconditions detected by the one or more methods includes at least one ofthe following:

-   -   Power failure (no power, or insufficient power)    -   Transducer fault detection    -   Failure to detect the presence of a component    -   Operating parameters outside recommended ranges (e.g. pressure,        flow, temperature, PaO₂)    -   Failure of a test alarm to generate a detectable alarm signal.

Upon detection of the fault condition, the corresponding algorithmsignals the presence of the fault by one or more of the following:

-   -   Initiation of an audible, visual &/or kinetic (e.g. vibrating)        alarm    -   Sending a message to an external device    -   Logging of the incident

Therapy Device 4245

In a preferred form of the present technology, the therapy device 4245is under the control of the control module 4330 to deliver therapy to apatient 1000.

Preferably the therapy device 4245 is a positive air pressure device4140.

HUMIDIFIER 5000

In one form of the present technology there is provided a humidifier5000 comprising a water reservoir 5110 and a heating plate 5120.

GLOSSARY

For purposes of the present technology disclosure, in certain forms ofthe present technology, one or more of the following definitions mayapply. In other forms of the present technology, alternative definitionsmay apply.

General

Air: In certain forms of the present technology, air supplied to apatient may be atmospheric air, and in other forms of the presenttechnology atmospheric air may be supplemented with oxygen.

Continuous Positive Airway Pressure (CPAP): CPAP treatment will be takento mean the application of a supply of air or breathable gas to theentrance to the airways at a pressure that is continuously positive withrespect to atmosphere, and preferably approximately constant through arespiratory cycle of a patient. In some forms, the pressure at theentrance to the airways will vary by a few centimetres of water within asingle respiratory cycle, for example being higher during inhalation andlower during exhalation. In some forms, the pressure at the entrance tothe airways will be slightly higher during exhalation, and slightlylower during inhalation. In some forms, the pressure will vary betweendifferent respiratory cycles of the patient, for example being increasedin response to detection of indications of partial upper airwayobstruction, and decreased in the absence of indications of partialupper airway obstruction.

Aspects of PAP devices

Air circuit: A conduit or tube constructed and arranged in use todeliver a supply of air or breathable gas between a PAP device and apatient interface. In particular, the air circuit may be in fluidconnection with the outlet of the pneumatic block and the patientinterface. The air circuit may be referred to as air delivery tube. Insome cases there may be separate limbs of the circuit for inhalation andexhalation. In other cases a single limb is used.

APAP: Automatic Positive Airway Pressure. Positive airway pressure thatis continually adjustable between minimum and maximum limits, dependingon the presence or absence of indications of SDB events.

Blower or flow generator: A device that delivers a flow of air at apressure above ambient pressure.

Controller: A device, or portion of a device that adjusts an outputbased on an input. For example one form of controller has a variablethat is under control the control variable—that constitutes the input tothe device. The output of the device is a function of the current valueof the control variable, and a set point for the variable. Aservo-ventilator may include a controller that has ventilation as aninput, a target ventilation as the set point, and level of pressuresupport as an output. Other forms of input may be one or more of oxygensaturation (SaO₂), partial pressure of carbon dioxide (PCO₂), movement,a signal from a photoplethysmogram, and peak flow. The set point of thecontroller may be one or more of fixed, variable or learned. Forexample, the set point in a ventilator may be a long term average of themeasured ventilation of a patient. Another ventilator may have aventilation set point that changes with time. A pressure controller maybe configured to control a blower or pump to deliver air at a particularpressure.

Therapy: Therapy in the present context may be one or more of positivepressure therapy, oxygen therapy, carbon dioxide therapy, control ofdead space, and the administration of a drug.

Motor: A device for converting electrical energy into rotary movement ofa member. In the present context the rotating member is an impeller,which rotates in place around a fixed axis so as to impart a pressureincrease to air moving along the axis of rotation.

Positive Airway Pressure (PAP) device: A device for providing a supplyof air at positive pressure to the airways.

Transducers: A device for converting one form of energy or signal intoanother. A transducer may be a sensor or detector for convertingmechanical energy (such as movement) into an electrical signal. Examplesof transducers include pressure sensors, flow sensors, carbon dioxide(CO₂) sensors, oxygen (O₂) sensors, effort sensors, movement sensors,noise sensors, a plethysmograph, and cameras.

Volute: The casing of the centrifugal pump that receives the air beingpumped by the impeller, slowing down the flow rate of air and increasingthe pressure. The cross-section of the volute increases in area towardsthe discharge port.

Aspects of the Respiratory Cycle

Apnea: An apnea will be said to have occurred when flow falls below apredetermined threshold for a duration, e.g. 10 seconds. An obstructiveapnea will be said to have occurred when, despite patient effort, someobstruction of the airway does not allow air to flow. A central apneawill be said to have occurred when an apnea is detected that is due to areduction in breathing effort, or the absence of breathing effort.

Breathing rate: The rate of spontaneous respiration of a patient,usually measured in breaths per minute.

Duty cycle: The ratio of inhalation time, Ti to total breath time, Ttot.

Effort (breathing): The work done by a spontaneously breathing personattempting to breathe.

Expiratory portion of a breathing cycle: The period from the start ofexpiratory flow to the start of inspiratory flow.

Flow limitation: Preferably, flow limitation will be taken to be thestate of affairs in a patient's respiration where an increase in effortby the patient does not give rise to a corresponding increase in flow.Where flow limitation occurs during an inspiratory portion of thebreathing cycle it may be described as inspiratory flow limitation.Where flow limitation occurs during an expiratory portion of thebreathing cycle it may be described as expiratory flow limitation.

Types of flow limited inspiratory waveforms:

-   -   (i) Flattened: Having a rise followed by a relatively flat        portion, followed by a fall.    -   (ii) Chair-shaped: Having a single local peak, the peak being at        the leading edge, followed by a relatively flat portion.    -   (iii) Reverse-chair shaped: Having a relatively flat portion        followed by single local peak, the peak being at the trailing        edge.    -   (iv) M-shaped: Having two local peaks, one at the leading edge,        and one at the trailing edge, and a relatively flat portion or a        dip between the two peaks.

Hypopnea: A hypopnea will be taken to be a reduction in flow, but not acessation of flow. In one form, a hypopnea may be said to have occurredwhen there is a reduction in flow below a threshold for a duration. Inone form in adults, the following either of the following may beregarded as being hypopneas:

-   -   (i) a 30% reduction in patient breathing for at least 10 seconds        plus an associated 4% desaturation; or    -   (ii) a reduction in patient breathing (but less than 50%) for at        least 10 seconds, with an associated desaturation of at least 3%        or an arousal.

Hyperpnea: An increase in flow to a level higher than normal flow.

Inspiratory portion of a breathing cycle: Preferably the period from thestart of inspiratory flow to the start of expiratory flow will be takento be the inspiratory portion of a breathing cycle.

Patency (airway): The degree of the airway being open, or the extent towhich the airway is open. A patent airway is open. Airway patency may bequantified, for example with a value of one (1) being patent, and avalue of zero (0), being closed.

Positive End-Expiratory Pressure (PEEP): The pressure above atmospherein the lungs that exists at the end of expiration.

Peak flow (Qpeak): The maximum value of flow during the inspiratoryportion of the respiratory flow waveform.

Respiratory flow, airflow, patient airflow, respiratory airflow (Qr):These synonymous terms may be understood to refer to the PAP device'sestimate of respiratory airflow, as opposed to “true respiratory flow”or “true respiratory airflow”, which is the actual respiratory flowexperienced by the patient, usually expressed in litres per minute.

Tidal volume (Vt): The volume of air inhaled or exhaled during normalbreathing, when extra effort is not applied.

(inhalation) Time (Ti): The duration of the inspiratory portion of therespiratory flow waveform.

(exhalation) Time (Te): The duration of the expiratory portion of therespiratory flow waveform.

(total) Time (Ttot): The total duration between the start of theinspiratory portion of one respiratory flow waveform and the start ofthe inspiratory portion of the following respiratory flow waveform.

Typical recent ventilation: The value of ventilation around which recentvalues over some predetermined timescale tend to cluster, that is, ameasure of the central tendency of the recent values of ventilation.

Upper airway obstruction (UAO): includes both partial and total upperairway obstruction. This may be associated with a state of flowlimitation, in which the level of flow increases only slightly or mayeven decrease as the pressure difference across the upper airwayincreases (Starling resistor behaviour).

Ventilation (Vent): A measure of the total amount of gas being exchangedby the patient's respiratory system, including both inspiratory andexpiratory flow, per unit time. When expressed as a volume per minute,this quantity is often referred to as “minute ventilation”. Minuteventilation is sometimes given simply as a volume, understood to be thevolume per minute.

PAP Device Parameters

Flow rate: The instantaneous volume (or mass) of air delivered per unittime. While flow rate and ventilation have the same dimensions of volumeor mass per unit time, flow rate is measured over a much shorter periodof time. Flow may be nominally positive for the inspiratory portion of abreathing cycle of a patient, and hence negative for the expiratoryportion of the breathing cycle of a patient. In some cases, a referenceto flow rate will be a reference to a scalar quantity, namely a quantityhaving magnitude only. In other cases, a reference to flow rate will bea reference to a vector quantity, namely a quantity having bothmagnitude and direction. Flow will be given the symbol Q. Total flow,Qt, is the flow of air leaving the PAP device. Vent flow, Qv, is theflow of air leaving a vent to allow washout of exhaled gases. Leak flow,Ql, is the flow rate of unintentional leak from a patient interfacesystem. Respiratory flow, Qr, is the flow of air that is received intothe patient's respiratory system.

Leak: A flow of air to the ambient. Leak may be intentional, for exampleto allow for the washout of exhaled CO₂. Leak may be unintentional, forexample, as the result of an incomplete seal between a mask and apatient's face.

Pressure: Force per unit area. Pressure may be measured in a range ofunits, including cm H₂O, g-f/cm², hectopascal. 1 cm H₂O is equal to Ig-f/cm² and is approximately 0.98 hectopascal. In this specification,unless otherwise stated, pressure is given in units of cm H₂O. For nasalCPAP treatment of OSA, a reference to treatment pressure is a referenceto a pressure in the range of about 4-20 cm H₂O, or about 4-30 cm H₂O.The pressure in the patient interface (or, more succinctly, maskpressure) is given the symbol Pm.

Sound Power: The energy per unit time carried by a sound wave. The soundpower is proportional to the square of sound pressure multiplied by thearea of the wavefront. Sound power is usually given in decibels SWL,that is, decibels relative to a reference power, normally taken as 10⁻¹²watt.

Sound Pressure: The local deviation from ambient pressure at a giventime instant as a result of a sound wave travelling through a medium.Sound power is usually given in decibels SPL, that is, decibels relativeto a reference power, normally taken as 20×10⁻⁶ pascal (Pa), consideredthe threshold of human hearing.

Terms for Ventilators

Adaptive Servo-Ventilator: A ventilator that has a changeable, ratherthan fixed target ventilation. The changeable target ventilation may belearned from some characteristic of the patient, for example, arespiratory characteristic of the patient.

Backup rate: a parameter of a ventilator that establishes the minimumrespiration rate (typically in number of breaths per minute) that theventilator will deliver to the patient, if not otherwise triggered.

Cycled: The termination of a ventilator's inspiratory phase. When aventilator delivers a breath to a spontaneously breathing patient, atthe end of the inspiratory portion of the breathing cycle, theventilator is said to be cycled to stop delivering the breath.

EPAP (or EEP): a base pressure, to which a pressure varying within thebreath is added to produce the desired mask pressure which theventilator will attempt to achieve at a given time.

IPAP: desired mask pressure which the ventilator will attempt to achieveduring the inspiratory portion of the breath.

Pressure support: A number that is indicative of the increase inpressure during ventilator inspiration over that during ventilatorexpiration, and generally means the difference in pressure between themaximum value during inspiration and the minimum value during expiration(e.g., PS=IPAP−EPAP). In some contexts pressure support means thedifference which the device aims to achieve, rather than what itactually achieves.

Servo-ventilator: A ventilator that measures patient ventilation has atarget ventilation, and which adjusts the level of pressure support tobring the patient ventilation towards the target ventilation.

Spontaneous/Timed (S/T)—A mode of a ventilator or other device thatattempts to detect the initiation of a breath of a spontaneouslybreathing patient. If however, the device is unable to detect a breathwithin a predetermined period of time, the device will automaticallyinitiate delivery of the breath.

Swing: Equivalent term to pressure support.

Triggered: When a ventilator delivers a breath of air to a spontaneouslybreathing patient, it is said to be triggered to do so at the initiationof the respiratory portion of the breathing cycle by the patient'sefforts.

Ventilator: A mechanical device that provides pressure support to apatient to perform some or all of the work of breathing.

Ventilator inspiration and ventilator expiration: the periods duringwhich the ventilator considers that it should deliver pressuresappropriate respectively to patient inspiration and expiration.Depending on the quality of patient-ventilator synchronisation, and thepresence of upper airway obstruction, these may or may not correspond toactual patient inspiration or expiration.

Anatomy of the Respiratory System

Diaphragm: A sheet of muscle that extends across the bottom of the ribcage. The diaphragm separates the thoracic cavity, containing the heart,lungs and ribs, from the abdominal cavity. As the diaphragm contractsthe volume of the thoracic cavity increases and air is drawn into thelungs.

Larynx: The larynx, or voice box houses the vocal folds and connects theinferior part of the pharynx (hypopharynx) with the trachea.

Lungs: The organs of respiration in humans. The conducting zone of thelungs contains the trachea, the bronchi, the bronchioles, and theterminal bronchioles. The respiratory zone contains the respiratorybronchioles, the alveolar ducts, and the alveoli.

Nasal cavity: The nasal cavity (or nasal fossa) is a large air filledspace above and behind the nose in the middle of the face. The nasalcavity is divided in two by a vertical fin called the nasal septum. Onthe sides of the nasal cavity are three horizontal outgrowths callednasal conchae (singular “concha”) or turbinates. To the front of thenasal cavity is the nose, while the back blends, via the choanae, intothe nasopharynx.

Pharynx: The part of the throat situated immediately inferior to (below)the nasal cavity, and superior to the oesophagus and larynx. The pharynxis conventionally divided into three sections: the nasopharynx(epipharynx) (the nasal part of the pharynx), the oropharynx(mesopharynx) (the oral part of the pharynx), and the laryngopharynx(hypopharynx).

Mathematical Terms

Fuzzy logic is used in a number of places in this technology. Thefollowing is used to indicate a fuzzy membership function, which outputsa “fuzzy truth value” in the range [0, 1], 0 representing fuzzy falseand 1 representing fuzzy true:

FuzzyMember (ActualQuantity, ReferenceQuantity1,FuzzyTruthValueAtReferenceQuantity1, ReferenceQuantity2,FuzzyTruthValueAtReferenceQuantity2, . . . , ReferenceQuantityN,FuzzyTruthValueAtReferenceQuantityN)

A fuzzy membership function is defined as

${{FuzzyMember}( {x,x_{1},f_{1},x_{2},f_{2},\ldots \mspace{14mu},x_{N},f_{N}} )} = \{ \begin{matrix}{f_{1},} & {x < x_{1}} \\{f_{N},} & {x \geq x_{N}} \\{{{InterpOnInterval}( {x,x_{k},f_{k},x_{k + 1},f_{k + 1}} )},} & {{x_{k} \leq x < x_{k + 1}},{1 \leq k \leq N}}\end{matrix} $

where

${{InterpOnInterval}( {x,x_{k},f_{k},x_{k + 1},f_{k + 1}} )} = \{ {\begin{matrix}{{f_{k} + \frac{( {f_{k + 1} - f_{k}} )( {x - x_{k}} )}{x_{k + 1} - x_{k}}},} & {x_{k} \neq x_{k + 1}} \\f_{k} & {otherwise}\end{matrix},} $

the f_(i) are fuzzy truth values, and x and the x_(j) are real numbers.

The function “Interp” is defined to be the same as “FuzzyMember”, exceptthat the values f_(k) are interpreted as real numbers rather than fuzzytruth values.

The fuzzy “Or” of fuzzy truth values is the maximum of those values; thefuzzy “And” of fuzzy truth values is the minimum of these values. Thesewill be indicated by the functions FuzzyOr and FuzzyAnd of two or morefuzzy truth values. It is to be understood that other typicaldefinitions of these fuzzy operations would work similarly in thepresent technology.

“Exponential decay towards zero” with a time constant r means thatduring any period of decay starting at time t=T, the value of thedecaying quantity V is given by

${V(t)} = {{V(T)}*{\exp ( {- \frac{t - T}{\tau}} )}}$

Advantages

The oscillations in central drive to the respiratory musculatureassociated with Cheyne-Stokes Respiration may be associated withoscillations in drive to the upper airway musculature, exacerbating anytendency to upper airway obstruction. Any method which attempts tocounteract the self-sustaining oscillations in respiratory drive byventilating the patient, typically with more ventilator drive duringperiods of low patient effort than during periods of high patienteffort, needs the upper airway to be substantially open when it isattempting to deliver ventilatory assistance, otherwise the ventilatoryassistance will be to some extent, and often totally, ineffective duringthe periods of low or zero patient effort, and thus unable to stabilisethe patient's ventilation.

This need to keep the upper airway open is typically addressed byattempting to set an expiratory positive airway pressure (EPAP) suchthat the upper airway is kept open at all times. This may be achieved bysome kind of iterative adjustment of EPAP while observing indicators ofthe patency of the airway at various EPAP levels, in a procedure calleda titration. Titration is a skilled and typically expensive operation,preferably being conducted in a sleep laboratory, and may not yield anEPAP sufficient to overcome upper airway obstruction (UAO). Reasons forthis include the fact that UAO is often postural, and the patient maynever during the titration night assume the posture which produces theworst UAO, typically the supine posture. Sedative and other drugs mayvariably influence the upper airway. There is also evidence that thedegree of cardiac failure affects the degree of upper airway obstructionvia oedema of the upper airway. Hence an exacerbation of cardiac failuremay worsen upper airway obstruction to an extent which cannot beanticipated during a titration night.

An advantage of the present technology is therefore the ability todiagnose and/or treat the combination of CSR and OSA at the patient'shome without the need for PSG and/or titration in a sleep laboratory.

A further advantage is the ability to treat the combination of CSR andOSA more effectively and in a manner that improves patient comfort.

In particular, an advantage is to counteract the tendency of automaticservo ventilators to inappropriately increase the target ventilation inresponse to artifacts such as uncompensated leak.

Other Remarks

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

Unless the context clearly dictates otherwise and where a range ofvalues is provided, it is understood that each intervening value, to thetenth of the unit of the lower limit, between the upper and lower limitof that range, and any other stated or intervening value in that statedrange is encompassed within the technology. The upper and lower limitsof these intervening ranges, which may be independently included in theintervening ranges, are also encompassed within the technology, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the technology.

Furthermore, where a value or values are stated herein as beingimplemented as part of the technology, it is understood that such valuesmay be approximated, unless otherwise stated, and such values may beutilized to any suitable significant digit to the extent that apractical technical implementation may permit or require it.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this technology belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present technology, a limitednumber of the exemplary methods and materials are described herein.

When a particular material is identified as being preferably used toconstruct a component, obvious alternative materials with similarproperties may be used as a substitute. Furthermore, unless specified tothe contrary, any and all components herein described are understood tobe capable of being manufactured and, as such, may be manufacturedtogether or separately.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include their plural equivalents,unless the context clearly dictates otherwise.

All publications mentioned herein are incorporated by reference todisclose and describe the methods and/or materials which are the subjectof those publications. The publications discussed herein are providedsolely for their disclosure prior to the filing date of the presentapplication. Nothing herein is to be construed as an admission that thepresent technology is not entitled to antedate such publication byvirtue of prior invention. Further, the dates of publication providedmay be different from the actual publication dates, which may need to beindependently confirmed.

Moreover, in interpreting the disclosure, all terms should beinterpreted in the broadest reasonable manner consistent with thecontext. In particular, the terms “comprises” and “comprising” should beinterpreted as referring to elements, components, or steps in anon-exclusive manner, indicating that the referenced elements,components, or steps may be present, or utilized, or combined with otherelements, components; or steps that are not expressly referenced.

The subject headings used in the detailed description are included onlyfor the ease of reference of the reader and should not be used to limitthe subject matter found throughout the disclosure or the claims. Thesubject headings should not be used in construing the scope of theclaims or the claim limitations.

Although the technology herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thetechnology. In some instances, the terminology and symbols may implyspecific details that are not required to practice the technology. Forexample, although the terms “first” and “second” may be used, unlessotherwise specified, they are not intended to indicate any order but maybe utilised to distinguish between distinct elements. Furthermore,although process steps in the methodologies may be described orillustrated in an order, such an ordering is not required. Those skilledin the art will recognize that such ordering may be modified and/oraspects thereof may be conducted concurrently or even synchronously.

It is therefore to be understood that numerous modifications may be madeto the illustrative embodiments and that other arrangements may bedevised without departing from the spirit and scope of the technology.

REFERENCE LABEL LIST

-   -   patient 1000    -   patient interface 3000    -   structure 3100    -   plenum chamber 3200    -   structure 3300    -   vent 3400    -   connection port 3600    -   pap device 4000    -   external housing 4010    -   upper portion 4012    -   portion 4014    -   panel 4015    -   chassi 4016    -   handle 4018    -   pneumatic block 4020    -   pneumatic component 4100    -   air filter 4110    -   inlet air filter 4112    -   outlet air filter 4114    -   muffler 4120    -   inlet muffler 4122    -   outlet muffler 4124    -   controllable pressure device 4140    -   controllable blower 4142    -   brushless DC motor 4144    -   back valve 4160    -   air circuit 4170    -   supplemental oxygen 4180    -   electrical component 4200    -   PCBA 4202    -   electrical power supply 4210    -   input device 4220    -   processor 4230    -   clock 4232    -   therapy device controller 4240    -   therapy device 4245    -   protection circuit 4250    -   memory 4260    -   transducer 4270    -   pressure transducer 4272    -   flow 4274    -   motor speed signal 4276    -   data communication interface 4280    -   remote external communication network 4282    -   local external communication network 4284    -   such remote external device 4286    -   local external device 4288    -   output device 4290    -   display driver 4292    -   display 4294    -   algorithm module 4300    -   processing module 4310    -   pressure compensation algorithm 4312    -   vent flow 4314    -   leak flow 4316    -   respiratory flow 4318    -   algorithm 4319    -   therapy engine module 4320    -   phase determination algorithm 4321    -   waveform determination algorithm 4322    -   ventilation determination 4323    -   algorithm 4324    -   algorithm 4325    -   algorithm 4326    -   algorithm 4327    -   target ventilation 4328    -   therapy parameter 4329    -   control module 4330    -   fault condition 4340    -   humidifier 5000    -   water reservoir 5110    -   heating plate 5120    -   humidifier controller 5250    -   method 7100    -   step 7110    -   step 7120    -   step 7130    -   step 7140    -   step 7150    -   method 7200    -   step 7210    -   step 7220    -   method 7300    -   first step 7310    -   step 7320    -   step 7330    -   step 7335    -   step 7340    -   step 7350    -   step 7360    -   step 7370    -   method 7400    -   step 7410    -   step 7415    -   state detection step 7420    -   Step 7425    -   step 7430    -   step 7435    -   step 7440    -   step 7445    -   step 7450    -   step 7455    -   step 7460    -   step 7470    -   step 7495    -   apnea detection method 7500    -   step 7510    -   step 7520    -   step 7530    -   step 7540    -   step 7550    -   step 7560    -   method 7600    -   step 7610    -   step 7620    -   step 7630    -   method 7700    -   step 7710    -   step 7720    -   step 7730    -   step 7735    -   Step 7740    -   step 7750    -   step 7760    -   step 7770    -   step 7780    -   step 7790    -   method 7800    -   step 7810    -   step 7820    -   step 7830    -   step 7840    -   step 7850    -   step 7860    -   step 7870    -   method 7900    -   step 7910    -   step 7920    -   step 7930    -   step 7940    -   step 7950    -   method 71000    -   step 71010    -   step 71020    -   step 71030    -   step 71040    -   step 71050    -   step 71060    -   step 71070    -   step 71080    -   method 71100    -   step 71110    -   step 71120    -   step 71130    -   step 71140    -   step 71150    -   step 71160    -   step 71170    -   step 71180    -   method 71200    -   implementation step 71210    -   step 71220    -   step 71230    -   step 71240    -   step 71250    -   step 71260    -   method 71300    -   step 71320    -   step 71330    -   step 71340    -   step 71350    -   step 71360    -   step 71370    -   method 71400    -   step 71410    -   step 71420    -   step 71430    -   next step 71440    -   step 71450    -   step 71460    -   method 71500    -   dependent filter 71510    -   step 71520    -   next step 71530    -   step 71540    -   current output sample step 71550    -   step 71560    -   step 71570    -   step 71580    -   ventilation filter 71590    -   method 71600    -   step 71610    -   step 71620    -   following step 71630    -   step 71640    -   step 71650    -   method 71700    -   step 71710    -   step 71720    -   step 71730    -   step 71740    -   step 71750    -   step 71760    -   step 71790    -   optional step 71795

1. A servo-ventilator configured to: continuously compute a targetventilation such that the target ventilation rises more slowly as ameasure of recent uncompensated leak increases, and control the pressureof a supply of air so as to achieve the target ventilation.
 2. Apparatusfor treating a respiratory disorder, configured to compute a measure oftypical recent ventilation such that a rate of adjustment of the measureof typical recent ventilation is reduced as a measure of recentuncompensated leak increases.
 3. Apparatus according to claim 2,configured to compute the measure of typical recent ventilation by:computing an update proportion that generally decreases as the measureof recent uncompensated leak increases; accumulating the updateproportion on receipt of each input sample; and computing an update tothe measure of typical recent ventilation only when the accumulatedupdate proportion exceeds one.
 4. Apparatus according to claim 3,wherein the update to the measure of typical recent ventilation iscomputed as a sum of input samples since the last update, weighted bythe update proportion at each sample.
 5. Apparatus according to claim 3or claim 4, further comprising re-initializing the accumulated updateproportion upon computing the update to the measure of typical recentventilation.
 6. Apparatus according to any of claims 3 to 5, wherein themeasure of typical recent ventilation is computed from a measure ofinstantaneous ventilation.
 7. Apparatus according to claim 6, furthercomprising applying a lowpass ventilation filter to the measure oftypical recent ventilation.
 8. Apparatus according to any of claims 3 to7, wherein the measure of typical recent ventilation is computed fromthe output of a lowpass ventilation filter that is applied to a measureof instantaneous ventilation.
 9. Apparatus for treating a respiratorydisorder, configured to compute a target ventilation from a product of ameasure of typical recent ventilation and a target fraction, wherein thetarget fraction is dependent on the recent pressure support. 10.Apparatus according to claim 9, wherein the target fraction is decreasedas the stability of recent pressure support increases.
 11. Apparatusaccording to claim 10, configured to compute the stability of recentpressure support as a fuzzy truth variable indicating the extent towhich the pressure support has been stable for a first recent period andalso for a second recent period substantially shorter than the firstrecent period.
 12. Apparatus according to claim 11, wherein thestability of pressure support over each period is computed based onorder statistics of pressure support computed over that period. 13.Apparatus according to claim 11, wherein the fuzzy truth variableincludes the extent to which an amount of pressure support above apredetermined minimum pressure support has been nontrivial over thesecond recent period.
 14. Apparatus according to claim 11, wherein thefuzzy truth variable includes the extent to which the maximum value ofpressure support above a predetermined minimum pressure support islarge.
 15. Apparatus for treating a respiratory disorder, configured tocompute a target ventilation from a product of a measure of typicalrecent ventilation and a target fraction, using a decreasing rateconstant that is increased dependent on the stability of recent pressuresupport.
 16. Apparatus according to claim 15, configured to compute thestability of recent pressure support as a fuzzy truth variableindicating the extent to which the pressure support has been stable fora first recent period and also for a second recent period substantiallyshorter than the first recent period.
 17. Apparatus according to claim16, wherein the stability of pressure support over each period iscomputed based on order statistics of pressure support computed overthat period.
 18. Apparatus according to claim 16 or claim 17, whereinthe decreasing rate constant is increased by a factor that generallyincreases with the value of the fuzzy truth variable.
 19. Apparatusaccording to claim 18, wherein the factor is one plus twice the value ofthe fuzzy truth variable.
 20. Apparatus for treating a respiratorydisorder, configured to compute a target ventilation from aninstantaneous ventilation, wherein the rate of increase of the targetventilation is bounded by an upper limit on an upward slew rate of thetarget ventilation.
 21. Apparatus according to any one of claim 9 andclaim 15 and claim 20, wherein the target ventilation is bounded belowby a predetermined minimum target ventilation.
 22. Apparatus fortreating a respiratory disorder, the apparatus being configured to:compute a duration of a detected episode of apnea or hypopnea, andadjust a value of expiratory positive airway pressure (EPAP) accordingto the computed duration, such that with increasing duration, theadjusted value of EPAP exponentially approaches a value that is greaterthan a maximum EPAP value.
 23. Apparatus according to claim 22, whereinthe rate constant of the exponential increase is decreased as the valuethat is greater than the maximum EPAP value increases.
 24. Apparatusaccording to claim 22 or claim 23, further comprising limiting theadjusted value of EPAP to be no more than the maximum EPAP value. 25.Apparatus according to any of claims 22 to 24, further comprisingdecaying the EPAP value toward a minimum EPAP value in the absence ofany detected episode of apnea or hypopnea.
 26. Apparatus for treating arespiratory disorder, the apparatus being configured to: compute ameasure of M-shaped inspiratory flow limitation, and increase anexpiratory positive airway pressure (EPAP) value according to thecomputed measure of M-shaped inspiratory flow limitation, such that theamount of increase is dependent on a ratio of breathwise ventilation totypical recent ventilation.
 27. Apparatus according to claim 26, furthercomprising decaying the EPAP value toward a minimum EPAP value if thecomputed measure of M-shaped inspiratory flow limitation is zero. 28.Apparatus according to claim 26 or claim 27, further comprising decayingthe EPAP value to a minimum EPAP value if the inspiratory duration isgreater than a threshold.
 29. Apparatus according to any of claims 26 to28, wherein the amount of increase generally decreases as the ratioincreases.
 30. Apparatus according to any of claims 26 to 29, whereinM-shaped inspiratory flow limitation is characterised by an inspiratoryflow waveform having two local peaks, one at the leading edge, and oneat the trailing edge, and a relatively flat portion or a dip between thetwo peaks.
 31. Apparatus for treating a respiratory disorder, theapparatus being configured to: compute a measure of reverse-chairness ofinspiratory flow limitation, and increase an expiratory positive airwaypressure (EPAP) value according to the computed measure ofreverse-chairness of inspiratory flow limitation, such that the amountof increase depends on the consistency of reverse-chairness betweencurrent and preceding breaths.
 32. Apparatus according to claim 31,wherein the consistency of reverse chairness is computed as a weightedgeometric mean of the measures of reverse chairness computed for thecurrent and preceding breaths.
 33. Apparatus according to claim 31 orclaim 32, wherein the amount of increase is proportional to a variablethat transitions between the measure of reverse chairness of the currentbreath and the consistency of reverse-chairness between the current andpreceding breaths as the current value of EPAP increases.
 34. Apparatusaccording to any of claims 31 to 33, wherein the amount of increasedecreases with an increasing amount of leak.
 35. Apparatus according toany of claims 31 to 34, wherein the amount of increase decreases with anincreasing value of current EPAP.
 36. Apparatus according to any ofclaims 31 to 35, further comprising decaying the EPAP value toward aminimum EPAP value if the computed measure of reverse-chairness ofinspiratory flow limitation is less than a threshold.
 37. Apparatusaccording to any of claims 31 to 36, wherein reverse-chairness ofinspiratory flow limitation is characterised by an inspiratory flowwaveform having a relatively flat portion followed by single local peak,the peak being at the trailing edge.
 38. Apparatus for treating arespiratory disorder, the apparatus being configured to: compute ameasure of inspiratory snore, detect expiratory snore, and increase anexpiratory positive airway pressure (EPAP) value according to themeasure of inspiratory snore, in the absence of expiratory snore. 39.Apparatus according to claim 38, wherein the amount of increase isdecreased as an amount of recent uncompensated leak increases. 40.Apparatus according to claim 39, wherein the amount of recentuncompensated leak is computed as a maximum value during the currentbreath of a fuzzy truth variable indicating the extent of recentuncompensated leak.
 41. Apparatus according to any of claims 38 to 40,further comprising decaying the EPAP value toward a minimum EPAP valuein the absence of inspiratory snore or in the presence of expiratorysnore.
 42. Apparatus for treating a respiratory disorder, the apparatusbeing configured to estimate a phase of a current breathing cycle of apatient, wherein a weight given to a standard rate of change in thephase estimate is dependent on an extent to which the patient hasrecently been achieving ventilation at or above a target ventilation.43. Apparatus according to claim 42, wherein the extent to which thepatient has recently been achieving ventilation at or above a targetventilation is computed as a fuzzy truth variable indicating the extentto which the current level of pressure support is large.
 44. Apparatusaccording to claim 43, wherein the fuzzy truth variable is computedrelative to boundaries that increase as the minimum value of pressuresupport increases.
 45. Apparatus for treating a respiratory disorder,the apparatus being configured to deliver pressure support to a patientat a value that is a combination of: a value of pressure support that issufficient to increase instantaneous ventilation to a targetventilation; and a value of pressure support that is sufficient toincrease gross alveolar ventilation to a target gross alveolarventilation.
 46. Apparatus for detecting a hypopnea in a patientdependent on: an extent to which pressure support being delivered to thepatient is large, and an extent to which a measure of absolute value ofairflow of the patient is small compared to a target absolute airflow.47. Apparatus according to claim 46, wherein the target absolute airflowis twice a current target ventilation.
 48. Apparatus according to claim46 or claim 47, wherein the measure of absolute airflow is computed asan output of a lowpass filter on the absolute value of airflow. 49.Apparatus for computing a measure of M-shaped inspiratory, flowlimitation of a patient based on a version of an inspiratory flowwaveform that is symmetrised around a location of a notch in aninspiratory flow waveform.
 50. Apparatus according to claim 49, furthercomprising computing the location of the notch as the location of a peakof a combination of convolutions of the inspiratory airflow waveformwith left and right halves of a V-shaped kernel.
 51. Apparatus accordingto claim 50, wherein the combination is a modified geometric mean. 52.Apparatus according to any of claims 49 to 51, wherein the measure ofM-shaped inspiratory flow limitation is an extent to which thesymmetrised waveform resembles a third harmonic function.
 53. Apparatusaccording to claim 52, wherein the measure of M-shaped inspiratory flowlimitation is computed as a ratio of a power in a third harmonic of thesymmetrised waveform to a sum of powers in first and third harmonics ofthe symmetrised waveform.
 54. Apparatus according to any of claims 49 to53, wherein M-shaped inspiratory flow limitation is characterised by theinspiratory flow waveform having two local peaks, one at the leadingedge, and one at the trailing edge, and a relatively flat portion or adip between the two peaks.
 55. Apparatus according to any of claims 52to 54, wherein the measure of M-shaped inspiratory flow limitation isset to zero if a measure of symmetry of the inspiratory flow waveformabout the notch location is below a threshold.
 56. Apparatus accordingto claim 55, wherein the measure of symmetry of the inspiratory flowwaveform is computed as a ratio of a lesser of the third harmoniccomponents of first and second halves of the symmetrised waveform to asum of absolute values of the components.
 57. Apparatus for computing ameasure of reverse chairness of inspiratory flow limitation of a patientdependent on the extent of recent uncompensated leak in the delivery ofairflow to the patient.
 58. Apparatus according to claim 57, wherein themeasure of reverse chairness is proportional to a generally decreasingfunction of a fuzzy truth variable indicating the extent of recentuncompensated leak.
 59. Apparatus according to claim 57 or claim 58,wherein reverse-chairness of inspiratory flow limitation ischaracterised by an inspiratory flow waveform having a relatively flatportion followed by single local peak, the peak being at the trailingedge.
 60. Apparatus for computing a measure of inspiratory snore in apatient to whom airflow is being delivered via an interface, theapparatus being configured to compute the measure as a mean over aninspiratory portion of a current breath of a difference between theoutput of a snore filter on an instantaneous interface pressure and athreshold that is dependent on the instantaneous interface pressure. 61.Apparatus according to claim 60, wherein the mean is weighted by aweighting function that is dependent on respiratory airflow of thepatient.
 62. Apparatus according to claim 61, wherein the weightingfunction is low at low respiratory airflow and at high respiratoryairflow.
 63. Apparatus according to any of claims 60 to 62, wherein thethreshold is generally increasing with increasing interface pressure.64. Apparatus for detecting expiratory snore in a patient to whomairflow is being delivered via an interface, using joint thresholds onduration and intensity of the output of a snore filter on aninstantaneous interface pressure during an expiratory portion of acurrent breath.
 65. Apparatus according to claim 64, wherein the jointthresholds are expressed in a predetermined critical function of snoreintensity.
 66. Apparatus according to claim 65, wherein thepredetermined critical snore function decreases generally withincreasing snore intensity.
 67. Apparatus according to claim 65 or claim66, being configured to detect expiratory snore based on a comparisonbetween a reverse cumulative distribution function of the intensity ofthe snore filter output and the predetermined critical snore function.68. Apparatus according to claim 67, wherein expiratory snore isdetected if the reverse cumulative distribution function is above thecritical reverse cumulative distribution function at any value of snoreintensity above a minimum intensity.
 69. Apparatus for treating arespiratory disorder, the apparatus comprising: a pressure deviceconfigured to deliver a supply of air at a positive pressure to theairway of a patient over a conduit and an interface; and a controllerconfigured to: compute a measure of typical recent ventilation of thepatient such that a rate of adjustment of the measure of typical recentventilation is reduced as a measure of recent uncompensated leak in theconduit and/or the interface increases; and control the pressure of thesupply of air delivered by the pressure device dependent on the computedmeasure of typical recent ventilation so as to treat the respiratorydisorder.
 70. Apparatus for detecting a respiratory condition in apatient, the apparatus comprising: one or more sensors configured toprovide signals representing respiratory parameters of the patient, anda processor configured to analyse the signals from the sensors so as todetect the respiratory condition, wherein: the respiratory condition isa hypopnea, the respiratory parameter is an absolute airflow, and thedetection is dependent on an extent to which pressure support beingdelivered to the patient is large, and an extent to which the absoluteairflow of the patient is small compared to a target absolute airflow.71. A method of treating a respiratory disorder, the method comprising:computing a measure of typical recent ventilation such that a rate ofadjustment of the measure of typical recent ventilation is reduced as ameasure of recent uncompensated leak increases.
 72. A method accordingto claim 71, wherein the computing comprises: computing an updateproportion that generally decreases as the measure of recentuncompensated leak increases; accumulating the update proportion onreceipt of each input sample; and computing an update to the measure oftypical recent ventilation only when the accumulated update proportionexceeds one.
 73. A computer readable storage medium having recordedthereon computer program code that is configured to cause a processor tocarry out a method of treating a respiratory disorder, the method beingin accordance with claim 71 or claim
 72. 74. A method of treating arespiratory disorder, the method comprising: computing a targetventilation from a product of a measure of typical recent ventilationand a target fraction, wherein the target fraction is dependent on therecent pressure support.
 75. A computer readable storage medium havingrecorded thereon computer program code that is configured to cause aprocessor to carry out a method of treating a respiratory disorder, themethod being in accordance with claim
 74. 76. A method of treating arespiratory disorder, the method comprising: computing a targetventilation from a product of a measure of typical recent ventilationand a target fraction, using a decreasing rate constant that isincreased dependent on the stability of recent pressure support.
 77. Acomputer readable storage medium having recorded thereon computerprogram code that is configured to cause a processor to carry out amethod of treating a respiratory disorder, the method being inaccordance with claim
 76. 78. A method of treating a respiratorydisorder, the method comprising: computing a target ventilation from aninstantaneous ventilation, wherein the rate of increase of the targetventilation is bounded by an upper limit on an upward slew rate of thetarget ventilation.
 79. A computer readable storage medium havingrecorded thereon computer program code that is configured to cause aprocessor to carry out a method of treating a respiratory disorder, themethod being in accordance with claim
 78. 80. Apparatus for treating arespiratory disorder, the method comprising: computing a duration of adetected episode of apnea or hypopnea, and adjusting a value ofexpiratory positive airway pressure (EPAP) according to the computedduration, such that with increasing duration, the adjusted value of EPAPexponentially approaches a value that is greater than a maximum EPAPvalue.
 81. A computer readable storage medium having recorded thereoncomputer program code that is configured to cause a processor to carryout a method of treating a respiratory disorder, the method being inaccordance with claim
 80. 82. A method of treating a respiratorydisorder, the method comprising: computing a measure of M-shapedinspiratory flow limitation, and increasing an expiratory positiveairway pressure (EPAP) value according to the computed measure ofM-shaped inspiratory flow limitation, such that the amount of increaseis dependent on a ratio of breathwise ventilation to typical recentventilation.
 83. A computer readable storage medium having recordedthereon computer program code that is configured to cause a processor tocarry out a method of treating a respiratory disorder, the method beingin accordance with claim
 82. 84. A method of treating a respiratorydisorder, the method comprising: computing a measure ofreverse-chairness of inspiratory flow limitation, and increasing anexpiratory positive airway pressure (EPAP) value according to thecomputed measure of reverse-chairness of inspiratory flow limitation,such that the amount of increase depends on the consistency ofreverse-chairness between current and preceding breaths.
 85. A computerreadable storage medium having recorded thereon computer program codethat is configured to cause a processor to carry out a method oftreating a respiratory disorder, the method being in accordance withclaim
 84. 86. A method of treating a respiratory disorder, the methodcomprising: computing a measure of inspiratory snore, detectingexpiratory snore, and increasing an expiratory positive airway pressure(EPAP) value according to the measure of inspiratory snore, in theabsence of expiratory snore.
 87. A computer readable storage mediumhaving recorded thereon computer program code that is configured tocause a processor to carry out a method of treating a respiratorydisorder, the method being in accordance with claim
 86. 88. A method oftreating a respiratory disorder, the method comprising: estimating aphase of a current breathing cycle of a patient, wherein a weight givento a standard rate of change in the phase estimate is dependent on anextent to which the patient has recently been achieving ventilation ator above a target ventilation.
 89. A computer readable storage mediumhaving recorded thereon computer program code that is configured tocause a processor to carry out a method of treating a respiratorydisorder, the method being in accordance with claim
 88. 90. A method oftreating a respiratory disorder, the method comprising: deliveringpressure support to a patient at a value that is a combination of: avalue of pressure support that is sufficient to increase instantaneousventilation to a target ventilation; and a value of pressure supportthat is sufficient to increase gross alveolar ventilation to a targetgross alveolar ventilation.
 91. A computer readable storage mediumhaving recorded thereon computer program code that is configured tocause a processor to carry out a method of treating a respiratorydisorder, the method being in accordance with claim
 90. 92. A method ofdetecting a hypopnea in a patient, the method comprising: detecting thehypopnea dependent on: an extent to which pressure support beingdelivered to the patient is large, and an extent to which a measure ofabsolute value of airflow of the patient is small compared to a targetabsolute airflow.
 93. A computer readable storage medium having recordedthereon computer program code that is configured to cause a processor tocarry out a method of detecting a hypopnea in a patient, the methodbeing in accordance with claim
 92. 94. A method of computing a measureof M-shaped inspiratory flow limitation of a patient based on a versionof the inspiratory flow waveform that is symmetrised around a locationof a notch in an inspiratory flow waveform.
 95. A computer readablestorage medium having recorded thereon computer program code that isconfigured to cause a processor to carry out a method of computing ameasure of M-shaped inspiratory flow limitation of a patient, the methodbeing in accordance with claim
 94. 96. A method of computing a measureof reverse chairness of inspiratory flow limitation of a patientdependent on the extent of recent uncompensated leak in the delivery ofairflow to the patient.
 97. A computer readable storage medium havingrecorded thereon computer program code that is configured to cause aprocessor to carry out a method of computing a measure of reversechairness of inspiratory flow limitation of a patient, the method beingin accordance with claim
 96. 98. A method of computing a measure ofinspiratory snore in a patient to whom airflow is being delivered via aninterface, the method comprising: computing a mean over an inspiratoryportion of a current breath of a difference between the output of asnore filter on an instantaneous interface pressure and a threshold thatis dependent on the instantaneous interface pressure.
 99. A computerreadable storage medium having recorded thereon computer program codethat is configured to cause a processor to carry out a method ofcomputing a measure of inspiratory snore in a patient to whom airflow isbeing delivered via an interface, the method the method being inaccordance with claim
 98. 100. A method of detecting expiratory snore ina patient to whom airflow is being delivered via an interface, themethod comprising: using joint thresholds on duration and intensity ofthe output of a snore filter on an instantaneous interface pressureduring an expiratory portion of a current breath.
 101. A computerreadable storage medium having recorded thereon computer program codethat is configured to cause a processor to carry out a method ofdetecting expiratory snore in a patient to whom airflow is beingdelivered via an interface, the method the method being in accordancewith claim
 100. 102. A method of treating a respiratory disorder, themethod comprising: delivering a supply of air at a positive pressure tothe airway of a patient over a conduit and an interface; computing ameasure of typical recent ventilation of the patient such that the rateof adjustment of the measure of typical recent ventilation is reduced asa measure of recent uncompensated leak in the conduit and/or theinterface increases; and controlling the pressure of the supply of airdependent on the computed measure of typical recent ventilation so as totreat the respiratory disorder.
 103. A computer readable storage mediumhaving recorded thereon computer program code that is configured tocause a processor to carry out a method of treating a respiratorydisorder, the method the method being in accordance with claim 102.